Microstructure of High Temperature Oxidation Resistant Hf6B10Si31C2N50 and Hf7B10Si32C2N44 Films

High-temperature oxidation resistant amorphous Hf6B10Si31C2N50 and Hf7B10Si32C2N44 films were deposited by reactive pulsed dc magnetron sputtering. To investigate the oxidation mechanism, the films were annealed up to 1500 °C in air. The evolved microstructures were studied by X-ray diffraction and transmission electron microscopy. A three-layered microstructure was developed upon exposure to high temperature. An oxidized layer formed at the top surface for both films consisting of monoclinic and/or orthorhombic m-/o-HfO2 nanoparticles embedded in an amorphous SiOx-based matrix. The as-deposited bottom layer of the films remained amorphous (Hf6B10Si31C2N50) or partially recrystallized (Hf7B10Si32C2N44) exhibiting a h-Si3N4 and HfCxN1−x distribution along with formation of t-HfO2 at its top section. The two layers were separated by a partially oxidized transition layer composed of nanocrystalline h-Si3N4 and tetragonal t-HfO2. The oxidation process initiates at the bottom/transition layer interface with oxidation of Hf-rich domains either in the amorphous structure or in HfCxN1−x nanoparticles resulting in t-HfO2 separated by Si3N4 domains. The second stage occurs at the oxidized/transition layer interface characterized by densely packed HfO2, Si3N4 and quartz SiO2 nanostructures that can act as a barrier for oxygen diffusion. The small t-HfO2 nanoparticles merge and transform into large m-/o-HfO2 while h-Si3N4 forms amorphous SiOx matrix. A similar oxidation mechanism was observed in both films despite the different microstructures developed.


Introduction
Ultra-high temperature ceramics (UHTCs) have been extensively studied in the recent years as they usually possess desirable properties such as high hardness, high melting point, superior oxidation and corrosion resistance at high temperatures and good thermal stability [1][2][3][4][5][6][7][8][9][10]. Among these materials, ZrB 2 -SiC and HfB 2 -SiC have demonstrated the most promising high temperature oxidation resistance because an oxide layer, consisting of a metal oxide skeleton and silicon oxide matrix form providing excellent protection against further oxygen permeation [5,7,[11][12][13][14][15][16][17]. A wide range of potential applications including critical frame structure of hypersonic vehicles, high-speed cutting tools, refractory linings and high-temperature electrodes and microelectronics would benefit from the use of UHTCs [5,7]. In our recent studies, we have successfully deposited Hf-B-Si-C coatings that exhibit high hardness, high electrical conductivity and good oxidation resistance up to 800 • C [18,19].

Materials and Methods
The Hf 7 B 10 Si 32 C 2 N 44 and Hf 6 B 10 Si 31 C 2 N 50 films were deposited utilizing a Balzers BAS 450 PM sputtering system (Balzers, Liechtenstein). The films were deposited on polished and ultrasonically pre-cleaned single-crystalline SiC substrates using reactive magnetron co-sputtering of Hf, B, Si and C from a single B 4 C-Hf-Si target in an Ar-N 2 gas mixture. The single B 4 C-Hf-Si target (127 mm × 254 mm) was prepared using a B 4 C plate (6 mm thick) overlapped by p-type Si and Hf stripes with fixed 30% B 4 C + 50% Si + 20% Hf fractions in the target erosion area. The target was driven by a pulsed dc power supply (Rübig MP120, Wels, Austria) operating at a frequency of 10 kHz and average power of 500 W. During the reactive sputtering, the voltage pulse duration of 50 µs is sufficiently short to avoid micro-arcing at the non-conductive layer formed on the B 4 C-Hf-Si target (for further details see [18,19,25]). The base pressure before the deposition was 1 × 10 −3 Pa. During the deposition, the total pressure of argon-nitrogen gas mixture was 0.5 Pa with a N 2 fraction in the mixture of 25% for Hf 7 B 10 Si 32 C 2 N 44 and 50% for Hf 6 B 10 Si 31 C 2 N 50 film. The target to substrate distance was 100 mm and the substrate temperature was maintained at 450 • C. The substrates were held at a floating potential (i.e., the method to prepare well densified films without applying substrate bias) to reduce the ion-induced internal stresses in the film and improve their application potential. The film composition was determined by Rutherford backscattering spectrometry (RBS) and elastic recoil detection (ERD) methods using a Van de Graaf generator with a linear electrostatic accelerator. The annealing of the films was performed using a symmetrical high resolution Setaram TAG 2400 system (Caluire, France) in synthetic atmospheric air (flow rate of 1 L/h) from room temperature up to 1500 • C. The films were heated at a rate of 10 • C/min, and cooled down at a rate of 30 • C/min, respectively.
The crystallographic structure of the annealed films was first studied by X-ray diffraction (XRD) θ-2θ measurements in a Bruker D8 Advance Diffractometer (Billerica, MA, USA) using a Cu Kα radiation at voltage of 40 kV and current of 40 mA. The detailed microstructure of the annealed films Coatings 2020, 10, 1170 3 of 17 was studied by TEM. Plan-view TEM samples were prepared by mechanical grinding and polishing, followed by dimpling using a Gatan Model 656 dimple grinder (Pleasanton, CA, USA) and Ar-ion milling in a Gatan Model 691 precision ion polishing system (PIPS). Cross-section TEM samples were prepared using focus-ion beam (FIB) in a FEI Strata 400 dual-beam system (Waltham, MA, USA) and further cleaned using PIPS. SAED, TEM and HRTEM images were recorded in a Hitachi H-9500 electron microscope (Tokyo, Japan) operated at 300 keV with a point resolution of 0.18 nm.
Coatings 2020, 10, x FOR PEER REVIEW 3 of 17 followed by dimpling using a Gatan Model 656 dimple grinder (Pleasanton, CA, USA) and Ar-ion milling in a Gatan Model 691 precision ion polishing system (PIPS). Cross-section TEM samples were prepared using focus-ion beam (FIB) in a FEI Strata 400 dual-beam system (Waltham, MA, USA) and further cleaned using PIPS. SAED, TEM and HRTEM images were recorded in a Hitachi H-9500 electron microscope (Tokyo, Japan) operated at 300 keV with a point resolution of 0.18 nm.

Microstructure of the Annealed Hf6B10Si31C2N50 Film
Overall Film Structure Figure 2a presents a cross-section TEM image of the annealed Hf6B10Si31C2N50 exhibiting a discrete three-layered structure: (i) a ~300 nm thick nanocomposite oxide layer (OL) with HfO2 nanoparticles dispersed in a rather amorphous (α) SiOx-based matrix on the top surface followed by (ii) a ~350 nm thick partially oxidized transition layer (TL) and (iii) a ~580 nm thick amorphous layer (AL). A thin (~50 nm in thickness) reaction zone (RZ) is also observed at the bottom resulting from the interaction of the film with the SiC substrate. Figure 2b is a SAED pattern taken from an area covering part of the SiC substrate, the RZ and a section of the AL. The SAED pattern presents a   Figure 2a presents a cross-section TEM image of the annealed Hf 6 B 10 Si 31 C 2 N 50 exhibiting a discrete three-layered structure: (i) a~300 nm thick nanocomposite oxide layer (OL) with HfO 2 nanoparticles dispersed in a rather amorphous (α) SiO x -based matrix on the top surface followed by (ii) a~350 nm thick partially oxidized transition layer (TL) and (iii) a~580 nm thick amorphous layer (AL). A thin (~50 nm in thickness) reaction zone (RZ) is also observed at the bottom resulting from the interaction of the film with the SiC substrate. Figure 2b is a SAED pattern taken from an  The top oxide layer exhibits nearly the same microstructure as in the annealed Hf7B23Si17C4N45 [25,27], Hf6B21Si19C4N47 and Hf7B23Si22C6N40 films [26,28] studied previously, whereas the layers underneath present different structures. Figure 2c presents a SAED pattern taken from the amorphous-like layer in Figure 2a showing a circular diffraction band with the inner radius of ~3.4 Å and the outer radius of ~2.1 Å indicating formation of short range ordered sub nanometer scale clusters within the range from ~2.1 to 3.4 Å. Figure 2d is a SAED pattern taken from the TL with a columnar structure appearance presenting diffraction rings 1, 2, 3, 4, 5, 6, 7 and 8 with a lattice spacing of 3.86, 3.32, 2.97, 2.58, 1.82, 1.60, 1.55 and 1.54 Å, respectively. The diffractions 1, 2, 6 and 8 were identified to be the (110), (200), (222) and (303) of h-Si3N4, respectively. The diffractions 3, 5 and 7 match the (101), (112) and (211) of t-HfO2, respectively. While the diffraction ring 4 possibly corresponds to (102) of h-Si3N4 or its superposition with the (110) of t-HfO2. It is noted that t-HfO2 can be stable not only at high temperatures but also at small particle sizes due to its low surface energy [25]. Figure 2d demonstrates the presence of h-Si3N4 and t-HfO2 phases in the TL in the annealed Hf6B10Si31C2N50 film.  Figure 3a is a zoom-in cross-section TEM image of the TL in the annealed film presenting vertically oriented zig-zag dark columnar structures separated by bright structures with lateral dimensions of ~10-20 nm. X-ray energy-dispersive spectroscopy (EDS) analysis showed that the dark districts are Hf and O concentrated, whereas the bright regions Si and N concentrated. Thus, TL possesses a complex structure that was further analyzed. The top oxide layer exhibits nearly the same microstructure as in the annealed Hf 7 B 23 Si 17 C 4 N 45 [25,27], Hf 6 B 21 Si 19 C 4 N 47 and Hf 7 B 23 Si 22 C 6 N 40 films [26,28] studied previously, whereas the layers underneath present different structures. Figure 2c presents a SAED pattern taken from the amorphous-like layer in Figure 2a showing a circular diffraction band with the inner radius of~3.4 Å and the outer radius of~2.1 Å indicating formation of short range ordered sub nanometer scale clusters within the range from~2.1 to 3.4 Å. Figure 2d is a SAED pattern taken from the TL with a columnar structure appearance presenting diffraction rings 1, 2, 3, 4, 5, 6, 7 and 8 with a lattice spacing of 3.86, 3.32, 2.97, 2.58, 1.82, 1.60, 1.55 and 1.54 Å, respectively. The diffractions 1, 2, 6 and 8 were identified to be the (110), (200), (222) and (303) of h-Si 3 N 4 , respectively. The diffractions 3, 5 and 7 match the (101), (112) and (211) of t-HfO 2 , respectively. While the diffraction ring 4 possibly corresponds to (102) of h-Si 3 N 4 or its superposition with the (110) of t-HfO 2 . It is noted that t-HfO 2 can be stable not only at high temperatures but also at small particle sizes due to its low surface energy [25]. Figure 2d demonstrates the presence of h-Si 3 N 4 and t-HfO 2 phases in the TL in the annealed Hf 6 B 10 Si 31 C 2 N 50 film.  Figure 3a is a zoom-in cross-section TEM image of the TL in the annealed film presenting vertically oriented zig-zag dark columnar structures separated by bright structures with lateral dimensions of 10-20 nm. X-ray energy-dispersive spectroscopy (EDS) analysis showed that the dark districts are Hf  Figure 3b is a HRTEM image taken from the area "x" in Figure 3a presenting a mixture of two phases. The dark grain possesses the same crystallographic orientation, and its fast Fourier transformation (FFT) presents a single crystal diffraction pattern of t-HfO2 along the (111) zone axis (inset in Figure 3b). The bright grains beside the dark t-HfO2 are Si3N4 grains as determined by HRTEM and FFT. The lattice fringes in the two Si3N4 grains with a spacing of 3.87 Å correspond to (110).
The top oxidized layer in the annealed Hf6B10Si31C2N50 film exhibited a similar microstructure to that reported previously [27,28]. The HfO2 nanoparticles are mostly spherical with their size varying from ~10 to ~35 nm and have a primary crystal structure of m-HfO2 and o-HfO2 regardless of the particle size. The spherical HfO2 nanoparticles are dispersed in amorphous α-SiO2. Figure 3c presents a typical HRTEM image of a [010] oriented o-HfO2 nanoparticle embedded in α-SiO2. Figure 3d is a HRTEM image from the amorphous layer in the annealed Hf6B10Si31C2N50 film presenting dark and bright contrast nano clusters of a size of a few nanometers. The darker regions likely correspond to Hf-rich areas, while the brighter ones to light element (Si, B, C, N) rich regions. Clusters of sub nanometer ordered structures were frequently observed in this layer.

Interface Structures
To understand the structure evolution in the different layers in the annealed Hf6B10Si31C2N50 film, we have studied the structure of the various interfaces using cross-section HRTEM.
Interface between OL and TL. In the annealed Hf6B10Si31C2N50 film, the top OL consists of spherical m-HfO2 and o-HfO2 nanoparticles with a size of ~10-35 nm dispersed in α-SiO2 matrix. The TL is composed of irregular shaped t-HfO2 and Si3N4 nano structures indicating significant structure transformations occurred at the interface: (i) from the much smaller t-HfO2 nanoparticles in the TL to the large m-HfO2/o-HfO2 in the OL and (ii) from the crystalline Si3N4 into α-SiO2. Figure 4a is a HRTEM image of the interface between the OL and TL showing presence of a large HfO2 nanoparticle (~20 nm) and α-SiO2 on the top-right within the OL. Also, numerous small particles are present on the bottom-left within the TL, among which three irregular shaped small HfO2 particles with a size of ~5 nm and two small irregular shaped Si3N4 particles of ~6 nm, as determined by using the lattice spacing. It is noted that within the TL, the size of HfO2 and Si3N4 nano particles near the interface is smaller than those away from the interface. Furthermore remarkably, curved or onion-like lattice fringes with a spacing of ~3.4 Å (marked by arrow heads) were formed in the boundaries between Si3N4 and Si3N4, and Si3N4 and HfO2 nano grains. The curved or onion-like lattice fringes with a spacing of ~3.4 Å can be determined to be the (101) β-SiO2 [27,28]. Such evidence of β-SiO2 formation   Figure 3b). The bright grains beside the dark t-HfO 2 are Si 3 N 4 grains as determined by HRTEM and FFT. The lattice fringes in the two Si 3 N 4 grains with a spacing of 3.87 Å correspond to (110).
The top oxidized layer in the annealed Hf 6 B 10 Si 31 C 2 N 50 film exhibited a similar microstructure to that reported previously [27,28]. The HfO 2 nanoparticles are mostly spherical with their size varying from~10 to~35 nm and have a primary crystal structure of m-HfO 2 and o-HfO 2 regardless of the particle size. The spherical HfO 2 nanoparticles are dispersed in amorphous α-SiO 2 . Figure 3c presents a typical HRTEM image of a [010] oriented o-HfO 2 nanoparticle embedded in α-SiO 2 . Figure 3d is a HRTEM image from the amorphous layer in the annealed Hf 6 B 10 Si 31 C 2 N 50 film presenting dark and bright contrast nano clusters of a size of a few nanometers. The darker regions likely correspond to Hf-rich areas, while the brighter ones to light element (Si, B, C, N) rich regions. Clusters of sub nanometer ordered structures were frequently observed in this layer.

Interface Structures
To understand the structure evolution in the different layers in the annealed Hf 6 B 10 Si 31 C 2 N 50 film, we have studied the structure of the various interfaces using cross-section HRTEM.
Interface between OL and TL. In the annealed Hf 6 B 10 Si 31 C 2 N 50 film, the top OL consists of spherical m-HfO 2 and o-HfO 2 nanoparticles with a size of~10-35 nm dispersed in α-SiO 2 matrix. The TL is composed of irregular shaped t-HfO 2 and Si 3 N 4 nano structures indicating significant structure transformations occurred at the interface: (i) from the much smaller t-HfO 2 nanoparticles in the TL to the large m-HfO 2 /o-HfO 2 in the OL and (ii) from the crystalline Si 3 N 4 into α-SiO 2 . Figure 4a is a HRTEM image of the interface between the OL and TL showing presence of a large HfO 2 nanoparticle (~20 nm) and α-SiO 2 on the top-right within the OL. Also, numerous small particles are present on the bottom-left within the TL, among which three irregular shaped small HfO 2 particles with a size of 5 nm and two small irregular shaped Si 3 N 4 particles of~6 nm, as determined by using the lattice spacing. It is noted that within the TL, the size of HfO 2 and Si 3 N 4 nano particles near the interface is smaller than those away from the interface. Furthermore remarkably, curved or onion-like lattice fringes with a spacing of~3.4 Å (marked by arrow heads) were formed in the boundaries between Si 3 N 4 and Si 3 N 4 , and Si 3 N 4 and HfO 2 nano grains. The curved or onion-like lattice fringes with a spacing of~3.4 Å can be determined to be the (101) β-SiO 2 [27,28]. Such evidence of β-SiO 2 formation between Si 3 N 4 nanoparticles shows initial oxidation of Si 3 N 4 as a reaction of Si 3 N 4 with O at the interface. The evidence also suggests that initially, the nucleation of β-SiO 2 breaks a large Si 3 N 4 nano grain into small irregular ones and simultaneously crashes down the large HfO 2 grains within TL. Thus, it is likely that melting of β-SiO 2 into α-SiO 2 facilitated the merging of the small HfO 2 nanoparticles forming large spherical HfO 2 particles.
Coatings 2020, 10, x FOR PEER REVIEW 6 of 17 between Si3N4 nanoparticles shows initial oxidation of Si3N4 as a reaction of Si3N4 with O at the interface. The evidence also suggests that initially, the nucleation of β-SiO2 breaks a large Si3N4 nano grain into small irregular ones and simultaneously crashes down the large HfO2 grains within TL. Thus, it is likely that melting of β-SiO2 into α-SiO2 facilitated the merging of the small HfO2 nanoparticles forming large spherical HfO2 particles. Interface between TL and AL. Within the TL in the annealed Hf6B10Si31C2N50 film, the lateral sizes of the dark HfO2 and bright Si3N4 structures remain uniform along the thickness of the layer. The interface between TL and the underneath AL shows a clear and discrete boundary as shown in Figure  2a. As oxygen diffuses through the TL to the TL/AL interface, oxidation of Hf-rich clusters and crystallization of Si3N4 crystalline take place. Figure 4b is a HRTEM image of the TL/AL interface presenting atomic structure of t-HfO2 and Si3N4 nano grains. Atomically sharp interfaces (marked by arrows) were formed between the t-HfO2 and/or Si3N4 grains and AL. This morphology clearly indicates the nucleation of these two phases at the interface. It should be noted that the dark and bright clusters in the AL are locally ordered metallic Hf-rich and light element rich (Si, B, C and N) structures, respectively. The former was oxidized forming t-HfO2, and the latter crystallized to Si3N4 structures indicating a rapid structural and composition transition by crossing the interface from AL to TL. Gradual Si3N4 and t-HfO2 phase separation, coalescence and alignment occurs resulting in the columnar appearance of the TL.
Interface between AL/SiC. Figure 4c is a HRTEM image taken from the interface between the AL and SiC substrate showing presence of a few large Si3N4 grains and a small HfB2 strip atomically attached to SiC substrate and to a small HfN grain on the top. HRTEM studies of numerous regions of the interlayer along the interface show that HfN and HfB2 phase are only minor structures in this zone and almost all of them are attached to the SiC substrate. The formation of the thin interlayer with primary Si3N4 and minor HfB2 and HfN phases close to the SiC substrate is most likely caused by a thermal transport effect [30] from the SiC substrate resulting in localized partial recrystallization.  Interface between TL and AL. Within the TL in the annealed Hf 6 B 10 Si 31 C 2 N 50 film, the lateral sizes of the dark HfO 2 and bright Si 3 N 4 structures remain uniform along the thickness of the layer. The interface between TL and the underneath AL shows a clear and discrete boundary as shown in Figure 2a. As oxygen diffuses through the TL to the TL/AL interface, oxidation of Hf-rich clusters and crystallization of Si 3 N 4 crystalline take place. Figure 4b is a HRTEM image of the TL/AL interface presenting atomic structure of t-HfO 2 and Si 3 N 4 nano grains. Atomically sharp interfaces (marked by arrows) were formed between the t-HfO 2 and/or Si 3 N 4 grains and AL. This morphology clearly indicates the nucleation of these two phases at the interface. It should be noted that the dark and bright clusters in the AL are locally ordered metallic Hf-rich and light element rich (Si, B, C and N) structures, respectively. The former was oxidized forming t-HfO 2 , and the latter crystallized to Si 3 N 4 structures indicating a rapid structural and composition transition by crossing the interface from AL to TL. Gradual Si 3 N 4 and t-HfO 2 phase separation, coalescence and alignment occurs resulting in the columnar appearance of the TL.
Interface between AL/SiC. primary Si 3 N 4 and minor HfB 2 and HfN phases close to the SiC substrate is most likely caused by a thermal transport effect [30] from the SiC substrate resulting in localized partial recrystallization.

Microstructure within the Film Plane
We have studied the multilayered complex structure in the annealed Hf 6 B 10 Si 31 C 2 N 50 film over large areas in the film plane using plan-view TEM.
Overall structure. Figure 5a is a TEM image of the plan-view top oxide layer obtained by removing the underneath layers and substrate. It presents distribution of the spherical HfO 2 nanoparticles (black dots) with a very narrow size range from~10 to 35 nm. It is quite interesting to note that the HfO 2 nanoparticles are not homogenously arranged within the film plane but densely packed along certain lines and loosely packed over the rest areas. The densely packed nanoparticles yield a trace pattern like the one exhibited by HfO 2 in the TL. Microstructure within the Film Plane We have studied the multilayered complex structure in the annealed Hf6B10Si31C2N50 film over large areas in the film plane using plan-view TEM.
Overall structure. Figure 5a is a TEM image of the plan-view top oxide layer obtained by removing the underneath layers and substrate. It presents distribution of the spherical HfO2 nanoparticles (black dots) with a very narrow size range from ~10 to 35 nm. It is quite interesting to note that the HfO2 nanoparticles are not homogenously arranged within the film plane but densely packed along certain lines and loosely packed over the rest areas. The densely packed nanoparticles yield a trace pattern like the one exhibited by HfO2 in the TL.   Figure 5a. Figure 5b presents a cross-section of the columnar structure observed in Figures 2a and 3a. Based on the cross-section and plan-view TEM images, the TL structure in a three-dimensional (3D) space can be restored. During annealing of the Hf6B10Si31C2N50 film, the vertical oriented columnar HfO2 nanostructures with a lateral size of a few nanometers were formed and the interval regions in between were filled with Si3N4 structures with lateral size from ~40 to ~80 nm which possibly consisting of numerous Si3N4 sub-grains with their boundaries occupied by HfO2 fine structures.    Figure 5a. Figure 5b presents a cross-section of the columnar structure observed in Figures 2a and 3a. Based on the cross-section and plan-view TEM images, the TL structure in a three-dimensional (3D) space can be restored. During annealing of the Hf 6 B 10 Si 31 C 2 N 50 film, the vertical oriented columnar HfO 2 nanostructures with a lateral size of a few nanometers were formed and the interval regions in between were filled with Si 3 N 4 structures with lateral size from~40 to~80 nm which possibly consisting of numerous Si 3 N 4 sub-grains with their boundaries occupied by HfO 2 fine structures.  Figure 5d shows a TEM image of the plan-view interface between the top OL and TL in which the spherical dark particles are from OL whereas the irregular shaped particles are from TL. The surrounding areas around the large spherical HfO 2 particles show a very clean homogeneous bright contrast revealing the growth of the HfO 2 is accomplished by consuming the surrounding HfO 2 via attraction and migration leaving Hf free surroundings. Such a growth process is also accompanied by firstly oxidizing the Si 3 N 4 to β-SiO 2 followed by further transition to α-SiO 2 . Figure 6 is a plan-view HRTEM image of an interval area of Si 3 N 4 between columnar t-HfO 2 in the TL. The inset FFT was calculated from the entire image. The inner spots present a nearly perfect hexagonal reciprocal pattern indicating the Si 3 N 4 grains in the image possess nearly identical orientation with their (001) direction perpendicular to the image. The HfO 2 nanoparticles are directly attached to the Si 3 N 4 without any transition layers or material in between. For instance, grain HfO 2 -1 has an atomically sharp interface or boundary with respect to the Si 3 N 4 -1 and Si 3 N 4 -2 on both sides and grain HfO 2 -2 directly bonds to the Si 3 N 4 -3 with an abrupt atomically sharp interface. The results indicate that the Si 3 N 4 phase within the interval area in TL could be either a single crystal grain or multi-grains possessing nearly the same orientation with the (001) parallel to the normal of the film plane and the boundaries were sealed with HfO 2 nanoparticles.
Coatings 2020, 10, x FOR PEER REVIEW 8 of 17 Figure 5d shows a TEM image of the plan-view interface between the top OL and TL in which the spherical dark particles are from OL whereas the irregular shaped particles are from TL. The surrounding areas around the large spherical HfO2 particles show a very clean homogeneous bright contrast revealing the growth of the HfO2 is accomplished by consuming the surrounding HfO2 via attraction and migration leaving Hf free surroundings. Such a growth process is also accompanied by firstly oxidizing the Si3N4 to β-SiO2 followed by further transition to α-SiO2. Figure 6 is a plan-view HRTEM image of an interval area of Si3N4 between columnar t-HfO2 in the TL. The inset FFT was calculated from the entire image. The inner spots present a nearly perfect hexagonal reciprocal pattern indicating the Si3N4 grains in the image possess nearly identical orientation with their (001) direction perpendicular to the image. The HfO2 nanoparticles are directly attached to the Si3N4 without any transition layers or material in between. For instance, grain HfO2-1 has an atomically sharp interface or boundary with respect to the Si3N4-1 and Si3N4-2 on both sides and grain HfO2-2 directly bonds to the Si3N4-3 with an abrupt atomically sharp interface. The results indicate that the Si3N4 phase within the interval area in TL could be either a single crystal grain or multi-grains possessing nearly the same orientation with the (001) parallel to the normal of the film plane and the boundaries were sealed with HfO2 nanoparticles. Structure transformation at the OL/TL interface. As stated earlier, crossing the interface between TL and OL, Si3N4 structure was firstly transformed into β-SiO2 onion structure followed by a rapid transition into α-SiO2. In the meantime, the smaller t-HfO2 nano particles joined and coarsened the m-/o-HfO2 particles. To understand such structure transitions, we have focused on the TL/OL interface structure. Figure 7a is a HRTEM image taken from the interface in a plan-view TEM foil presenting a few Si3N4 and HfO2 grains. Formation of a β-SiO2 between two t-HfO2 grains was revealed. Within the TL, the t-HfO2 particles are separated by Si3N4. The observation of the β-SiO2 between two t-HfO2 particles indicates the transformation of Si3N4 to β-SiO2. Figure 7a also shows that the curved lattice fringes of β-SiO2 are directly attached to two Si3N4 grains on the right side providing strong evidence that oxidation of Si3N4 is linked to its transformation to β-SiO2. Structure transformation at the OL/TL interface. As stated earlier, crossing the interface between TL and OL, Si 3 N 4 structure was firstly transformed into β-SiO 2 onion structure followed by a rapid transition into α-SiO 2 . In the meantime, the smaller t-HfO 2 nano particles joined and coarsened the m-/o-HfO 2 particles. To understand such structure transitions, we have focused on the TL/OL interface structure. Figure 7a is a HRTEM image taken from the interface in a plan-view TEM foil presenting a few Si 3 N 4 and HfO 2 grains. Formation of a β-SiO 2 between two t-HfO 2 grains was revealed. Within the TL, the t-HfO 2 particles are separated by Si 3 N 4 . The observation of the β-SiO 2 between two t-HfO 2 Coatings 2020, 10, 1170 9 of 17 particles indicates the transformation of Si 3 N 4 to β-SiO 2 . Figure 7a also shows that the curved lattice fringes of β-SiO 2 are directly attached to two Si 3 N 4 grains on the right side providing strong evidence that oxidation of Si 3 N 4 is linked to its transformation to β-SiO 2 . Figure 7b is a HRTEM image presenting an incomplete/growing spherical o-HfO2 nanoparticle with attachments of β-SiO2 and t-HfO2 structures. The lattice fringes in the small t-HfO2 grain have a spacing of 2.95 Å, which is also close to the lattice space of (211) of o-HfO2. The attachment of the small t-HfO2 grain to the large o-HfO2 appears via a connection of a β-SiO2 structure in between. The evidence suggests that "melting" β-SiO2 to amorphous α-SiO2 facilitates the coalescence of the t-HfO2 grain with the large o-HfO2 progressing its growth process. Figure 7c is a HRTEM image showing a near perfect β-SiO2 onion structure overlaid on a large HfO2 spherical particle. The dark core of the onion structure presents lattice fringes with a spacing of 2.08 Å that can be identified as the (212) of o-HfO2 structure. This result suggests that the β-SiO2 onion structure plays an additional catalytic role in the growth of HfO2 particles in addition to the heat resistance role reported previously [27,28].

Microstructure of the Annealed Hf7B10Si32C2N44 Film
Overall Film Structure Figure 8 shows a cross-section TEM image of the annealed Hf7B10Si32C2N44 film presenting a nanocomposite oxide layer on the top (~300 nm thick) followed by a transition layer (~300 nm thick) and a partially recrystallized layer (RL). Figure 9 presents zoom-in TEM images taken from different regions in the annealed film. The top OL, Figure 9a, possesses similar microstructure characteristics to the annealed Hf6B10Si31C2N50 film in Figure 2a, the annealed Hf7B23Si17C4N45 [25,27], Hf6B21Si19C4N47 and Hf7B23Si22C6N40 films [26,28] studied previously. The TL has a similar appearance to that in the Hf6B10Si31C2N50 film, Figure 3a, but with more discontinuities in the dispersed HfO2 structures in the SiOx-based matrix, Figure 9b. The RL exhibits a uniform nanocomposite structure of dark and bright domains at its bottom, Figure 9c, and top, Figure 9d, sections. A similar, very thin interaction zone with the SiC substrate was also observed in this annealed film, Figure 8, containing Si3N4, small HfN phases and HfB2 strips atomically attached to the SiC substrate. Figure 7b is a HRTEM image presenting an incomplete/growing spherical o-HfO 2 nanoparticle with attachments of β-SiO 2 and t-HfO 2 structures. The lattice fringes in the small t-HfO 2 grain have a spacing of 2.95 Å, which is also close to the lattice space of (211) of o-HfO 2 . The attachment of the small t-HfO 2 grain to the large o-HfO 2 appears via a connection of a β-SiO 2 structure in between. The evidence suggests that "melting" β-SiO 2 to amorphous α-SiO 2 facilitates the coalescence of the t-HfO 2 grain with the large o-HfO 2 progressing its growth process. Figure 7c is a HRTEM image showing a near perfect β-SiO 2 onion structure overlaid on a large HfO 2 spherical particle. The dark core of the onion structure presents lattice fringes with a spacing of 2.08 Å that can be identified as the (212) of o-HfO 2 structure. This result suggests that the β-SiO 2 onion structure plays an additional catalytic role in the growth of HfO 2 particles in addition to the heat resistance role reported previously [27,28].

Microstructure of the Annealed Hf 7 B 10 Si 32 C 2 N 44 Film
Overall Film Structure Figure 8 shows a cross-section TEM image of the annealed Hf 7 B 10 Si 32 C 2 N 44 film presenting a nanocomposite oxide layer on the top (~300 nm thick) followed by a transition layer (~300 nm thick) and a partially recrystallized layer (RL). Figure 9 presents zoom-in TEM images taken from different regions in the annealed film. The top OL, Figure 9a, possesses similar microstructure characteristics to the annealed Hf 6 B 10 Si 31 C 2 N 50 film in Figure 2a, the annealed Hf 7 B 23 Si 17 C 4 N 45 [25,27], Hf 6 B 21 Si 19 C 4 N 47 and Hf 7 B 23 Si 22 C 6 N 40 films [26,28] studied previously. The TL has a similar appearance to that in the Hf 6 B 10 Si 31 C 2 N 50 film, Figure 3a, but with more discontinuities in the dispersed HfO 2 structures in the SiO x -based matrix, Figure 9b. The RL exhibits a uniform nanocomposite structure of dark and bright domains at its bottom, Figure 9c, and top, Figure 9d, sections. A similar, very thin interaction zone with the SiC substrate was also observed in this annealed film, Figure 8 [29]. The lattice constant of HfCxN1−x can be obtained using a Hf(CN) = 1-x ·a HfN + x·a HfC , for example, the lattice constant of HfC0.5N0.5 calculated using this formula is 4.582 Å, almost identical to the value reported (PDF #:02-2469, a = 4.586 Å, Fm3m) [29]. The diffraction spots 7 and 12 have a lattice spacing of 2.52 and 1.79 Å and can match the (110) and (200) t-HfO2, respectively. The appearance of highly converged spots 1, 2, 3, 4 and 5, rather than diffraction rings indicate formation of a highly textured Si3N4 structure within the bottom part of the RL. The presence of dense diffraction rings 6, 8 and 13     [29]. The lattice constant of HfCxN1−x can be obtained using a Hf(CN) = 1-x ·a HfN + x·a HfC , for example, the lattice constant of HfC0.5N0.5 calculated using this formula is 4.582 Å, almost identical to the value reported (PDF #:02-2469, a = 4.586 Å, Fm3m) [29]. The diffraction spots 7 and 12 have a lattice spacing of 2.52 and 1.79 Å and can match the (110) and (200) t-HfO2, respectively. The appearance of highly converged spots 1, 2, 3, 4 and 5, rather than diffraction rings indicate formation of a highly textured Si3N4 structure within the bottom part of the RL. The presence of dense diffraction rings 6, 8 and 13   [29]. The lattice constant of HfC x N 1−x can be obtained using a Hf(CN) = (1 − x) · a HfN + x · a HfC , for example, the lattice constant of HfC 0.5 N 0.5 calculated using this formula is 4.582 Å, almost identical to the value reported (PDF #:02-2469, a = 4.586 Å, Fm3m) [29]. The diffraction spots 7 and 12 have a lattice spacing of 2.52 and 1.79 Å and can match the (110) and (200) t-HfO 2 , respectively. The appearance of highly converged spots 1, 2, 3, 4 and 5, rather than diffraction rings indicate formation of a highly textured Si 3 N 4 structure within the bottom part of the RL. The presence of dense diffraction rings 6, 8 and 13 indicates formation of significant HfC x N 1−x structure. The diffraction that can correspond to (110) t-HfO 2 occupied only a very limited fraction of the diffraction ring 7 indicating a negligible amount (if any) of HfO 2 might have formed in this layer. Therefore, the microstructure of the bottom section of the RL (~300 nm thick layer above the substrate) involves a dominant orientation preferred h-Si 3 N 4 phase with dispersion of HfC x N 1−x nano phases.  The SAED patterns taken from the middle and top sections (300-600 nm and 600-900 nm sublayer above the substrate, respectively) of the RL are nearly identical as presented in Figure 10b. However, they show some differences compared to the pattern in Figure 10a as outlined in the following: (i) diffractions (100), (101), (110) and (200) of Si3N4 appear as weak diffraction rings 1-4 with many dots in Figure 10b rather than highly converged spots in Figure 10a, indicating disappearance of the Si3N4 texture structure or breakage of the larger Si3N4 grains into small grains in this area; (ii) the density and intensity of diffractions (111), (200) and (220) of HfCxN1−x on rings 6, 8 and 13 in Figure 10b is slightly decreased compared to those in Figure 10a indicating a slight reduction of the amount of HfCxN1−x phase within this sublayer compared to the region 0-300 nm above the substrate; (iii) more diffraction spots on ring 7 in Figure 10b compared to Figure 10a can indicate some formation of HfO2 within this area. Figure 10c shows a SAED pattern taken from the top section of RL (900-1200 nm above the substrate). The diffractions (100) Based on above studies, the microstructure within the RL and TL in the annealed Hf7B10Si32C2N44 film can be summarized as follows: (i) the RL bottom section (within 300 nm from the SiC substrate) The SAED patterns taken from the middle and top sections (300-600 nm and 600-900 nm sublayer above the substrate, respectively) of the RL are nearly identical as presented in Figure 10b. However, they show some differences compared to the pattern in Figure 10a as outlined in the following:  Figure 10b is slightly decreased compared to those in Figure 10a indicating a slight reduction of the amount of HfC x N 1−x phase within this sublayer compared to the region 0-300 nm above the substrate; (iii) more diffraction spots on ring 7 in Figure 10b compared to Figure 10a can indicate some formation of HfO 2 within this area. Figure 10c shows a SAED pattern taken from the top section of RL (900-1200 nm above the substrate). The diffractions (100) show similar characteristics to those in Figure 10c, whereas the rings 6, 8 and 13 of HfC x N 1−x structure are nearly absent except of very few leftovers and the rings 5, 7 and 12 are significantly enhanced. This result indicates that the majority of HfC x N 1−x within the TL has transformed to HfO 2 .
Based on above studies, the microstructure within the RL and TL in the annealed Hf 7 B 10 Si 32 C 2 N 44 film can be summarized as follows: (i) the RL bottom section (within 300 nm from the SiC substrate) consists of large Si 3 N 4 grains with preferred orientation surrounding HfC Microstructure of the TL and RL Sections Figure 11a shows a typical HRTEM image of the primary structure within the TL presenting three Si 3 N 4 grains and three t-HfO 2 grains, corresponding respectively, to the bright and dark areas in Figure 9b. The three distinct Si 3 N 4 grains with a size of~10-15 nm possess nearly the same (001) crystallographic orientation, the two t-HfO 2 particles present (101) lattice fringes with a spacing of 2.95 Å and the other one presents (110) t-HfO 2 fringes with a spacing of 2.52 Å. We found that most of the dark districts in Figure 9b are t-HfO 2 , while HfC x N 1−x type particles were rarely observed (not shown here). Microstructure of the TL and RL Sections Figure 11a shows a typical HRTEM image of the primary structure within the TL presenting three Si3N4 grains and three t-HfO2 grains, corresponding respectively, to the bright and dark areas in Figure 9b. The three distinct Si3N4 grains with a size of ~10-15 nm possess nearly the same (001) crystallographic orientation, the two t-HfO2 particles present (101) lattice fringes with a spacing of 2.95 Å and the other one presents (110) t-HfO2 fringes with a spacing of 2.52 Å. We found that most of the dark districts in Figure 9b are t-HfO2, while HfCxN1−x type particles were rarely observed (not shown here).  Figure 11b shows a HRTEM image of a typical structure of the RL bottom section (~300 nm above the substrate) presenting a complete Si3N4 grain oriented along the (111) with a size of ~20 nm and a partial Si3N4 grain with (101) lattice fringes along with two small HfCxN1−x grains of a size of ~7-8 nm with presence of (200) lattice fringes with a spacing of 2.26 Å. Note that this (200) lattice spacing is smaller from previous detected HfCxN1−x (2.31 Å for x = 0.7) phase approaching that of a binary HfN phase structure suggesting the stoichiometry of the HfCxN1−x varies across the thickness of the coating. The Si3N4 and HfCxN1−x grains correspond to the bright and dark districts in Figure 9d, respectively. Intensive HRTEM studies showed that the dark districts in Figure 9d correspond to HfCxN1−x type structure. Figure 11c shows a representative HRTEM image of the structure of the top RL section (~1200 nm above the substrate) presenting coexistence of one HfCxN1−x, two t-HfO2 and three Si3N4 grains.  Figure 11b shows a HRTEM image of a typical structure of the RL bottom section (~300 nm above the substrate) presenting a complete Si 3 N 4 grain oriented along the (111) with a size of~20 nm and a partial Si 3 N 4 grain with (101) lattice fringes along with two small HfC x N 1−x grains of a size of~7-8 nm with presence of (200) lattice fringes with a spacing of 2.26 Å. Note that this (200) lattice spacing is smaller from previous detected HfC x N 1−x (2.31 Å for x = 0.7) phase approaching that of a binary HfN phase structure suggesting the stoichiometry of the HfC x N 1−x varies across the thickness of the coating. The Si 3 N 4 and HfC x N 1−x grains correspond to the bright and dark districts in Figure 9d, respectively. Intensive HRTEM studies showed that the dark districts in Figure 9d correspond to HfC x N 1−x type structure. Figure 11c shows a representative HRTEM image of the structure of the top RL section (~1200 nm above the substrate) presenting coexistence of one HfC x N 1−x , two t-HfO 2 and three Si 3 N 4 grains. The HfC x N 1−x grain has a size of~5-7 nm presenting a 2D atomic image of the (001) zone. The two t-HfO 2 particles have a size of a few nanometers with presence of (101) lattice fringes. Among the three Si 3 N 4 grains, the one presenting (110) lattice fringes with a spacing of 3.87 Å is significantly larger than the other two with presence of (201) fringes of a spacing 2.88 Å and of (101) which have a size of~5-7 nm. This indicates that in this section, some of large Si 3 N 4 grains broke into smaller ones. The coexistence of HfC x N 1−x , t-HfO 2 and Si 3 N 4 structure was frequently observed in the top RL section in agreement with the electron diffraction analysis in Figure 10c. Transformation from HfCN to t-HfO 2 is feasible by primarily replacing C and N atoms with O without dramatically changing the Hf lattice.
Structure Transformations at TL/OL Interface Figure 12 is a HRTEM image taken from the interface between the oxide and the transition layer presenting a large HfO 2 at the interface. It is interesting to note that the upper half of the large HfO 2 particle in the OL side appears as a fully grown spherical structure covered by α-SiO 2 . However, the bottom half of the particle in the TL side was captured to be still in the coarsening stage. It is more interesting to note that a mixture of much smaller phases of Si 3 N 4 , β-SiO 2 , and HfO 2 are attached or being in the immediate vicinity of the growing side. The coalescence/attachment of the smaller HfO 2 grains from the TL to the large HfO 2 seems to be directly accommodated via the Si 3 N 4 and β-SiO 2 phases. Transformation from Si 3 N 4 to β-SiO 2 and to amorphous α-SiO 2 facilitates the approach and eventual consolidation of the smaller HfO 2 grains to the large growing HfO 2 to complete its growth. This transformation at the RL/OL interface is very similar to the one observed for the Hf 6 B 10 Si 31 C 2 N 50 film, Figure 7b. Thus, besides the different microstructures evolved in the bottom layer (amorphous vs. partially recrystallized) in the two films, the oxidation mechanism is very similar. grains from the TL to the large HfO2 seems to be directly accommodated via the Si3N4 and β-SiO2 phases. Transformation from Si3N4 to β-SiO2 and to amorphous α-SiO2 facilitates the approach and eventual consolidation of the smaller HfO2 grains to the large growing HfO2 to complete its growth. This transformation at the RL/OL interface is very similar to the one observed for the Hf6B10Si31C2N50 film, Figure 7b. Thus, besides the different microstructures evolved in the bottom layer (amorphous vs. partially recrystallized) in the two films, the oxidation mechanism is very similar.

Microstructure Evolution and Oxidation Mechanism
The present Hf6B10Si31C2N50 and Hf7B10Si32C2N44 and previously studied Hf7B23Si17C4N45, Hf6B21Si19C4N47 and Hf7B23Si22C6N40 films exhibit high oxidation resistance even up to 1500 °C in air [27,28]. All five HfBSiCN films annealed up to 1500 °C in air exhibit a very similar fully oxidized top layer with HfO2 nanoparticles dispersed in a dense amorphous SiOx matrix. However, the original as-deposited bottom layer of the annealed HfBSiCN films exhibits a structure that depends on film composition. The bottom layer in high N and low Si:B ratio (~1:1) films such as Hf7B23Si17C4N45 [27] and Hf6B21Si19C4N47 [28] remains amorphous. However, in low N (Hf7B23Si22C6N40) or high Si:B (~3:1) ratio (Hf7B10Si32C2N44) films, the bottom layer is partially recrystallized forming HfB2, HfN and Si3N4. Film Hf6B10Si31C2N50 falls in between the above two types of films since it has high N content and high

Microstructure Evolution and Oxidation Mechanism
The present Hf 6 B 10 Si 31 C 2 N 50 and Hf 7 B 10 Si 32 C 2 N 44 and previously studied Hf 7 B 23 Si 17 C 4 N 45 , Hf 6 B 21 Si 19 C 4 N 47 and Hf 7 B 23 Si 22 C 6 N 40 films exhibit high oxidation resistance even up to 1500 • C in air [27,28]. All five HfBSiCN films annealed up to 1500 • C in air exhibit a very similar fully oxidized top layer with HfO 2 nanoparticles dispersed in a dense amorphous SiO x matrix. However, the original as-deposited bottom layer of the annealed HfBSiCN films exhibits a structure that depends on film composition. The bottom layer in high N and low Si:B ratio (~1:1) films such as Hf 7 B 23 Si 17 C 4 N 45 [27] and Hf 6 B 21 Si 19 C 4 N 47 [28] remains amorphous. However, in low N (Hf 7 B 23 Si 22 C 6 N 40 ) or high Si:B (~3:1) ratio (Hf 7 B 10 Si 32 C 2 N 44 ) films, the bottom layer is partially recrystallized forming HfB 2 , HfN and Si 3 N 4 . Film Hf 6 B 10 Si 31 C 2 N 50 falls in between the above two types of films since it has high N content and high Si:B ratio. Indeed, the bottom layer in this film is mainly amorphous (high N effect) but with presence of short range ordered structures, Figure 2c, composed of sub nanometer ordered structures, Figure 3d (high Si:B ratio effect).
Thus, our results suggest that the high N content and Si:B ratio exercise opposite effects on the stability of the original as-deposited amorphous bottom layer. This effect is demonstrated in the present two new films that both contain a high Si:B ratio (~3:1) but present an entirely different bottom layer structure. A high N content in SiBCN films has been shown to promote amorphization by suppressing Si-Si bonds in the film structure [20,22,23]. This is especially evident in low Si containing films where the excess N is enough to maintain an amorphous structure. However, in high Si:B ratio films, the bottom layer structure is determined by the relative balance between these two effects. For example, in the Hf 7 B 10 Si 32 C 2 N 44 film, the Si:N ratio is very close to 3:4 required for Si 3 N 4 formation and as such there is no additional N to maintain the amorphous structure in the remaining of the film. Thus, this film is partially recrystallized. On the other hand, the Hf 6 B 10 Si 31 C 2 N 50 film has excess N (beyond Si 3 N 4 formation) thus, resisting recrystallization. However, it develops bright and dark contrast of atomic low ordered clusters indicative of the very early stage of recrystallization, Figure 3d. Thus, while the two bottom layer structures are different, they are also similar since they both show atomic rearrangements that are extensive in the partially recrystallized Hf 7 B 10 Si 32 C 2 N 44 film and in the first stage of nucleation in the high N content Hf 6 B 10 Si 31 C 2 N 50 film.
The above atomic structure of the bottom layer in the present two new films has a consequence in their oxidation behavior. They both show a TL that was absent in the oxidized microstructure of the previously studied low Si content films. The presence of the TL is due to their high Si content resulting in the observed extensive (recrystallization) or limited (small ordered atomic clustering) atomic scale separation. The presence of high Si in these two films ties up most (or all) of the N leaving Hf rich clusters that can be easily oxidized producing the TL. The Hf 6 B 10 Si 31 C 2 N 50 film exhibits a distinct AL/TL interface, Figure 2a, since both Si 3 N 4 nucleation and HfO 2 formation (oxidation) occur at the interface. The TEM observations show that the Hf 7 B 10 Si 32 C 2 N 44 film does not exhibit a clear RL/TL interface. The bottom layer in this film has been partially recrystallized into phases Si 3 N 4 and HfC x N 1−x at its bottom section with increasing amount of HfO 2 as its upper section is approached. Since the oxidation of HfC x N 1−x to HfO 2 occurs gradually no clear interface is produced. The TL is the section where all (or the majority) of the HfC x N 1−x phase has been oxidized to form HfO 2 . Thus, the TL in this film is also composed of Si 3 N 4 and HfO 2 nanostructures like those in the Hf 6 B 10 Si 31 C 2 N 50 film. It should be noted that a TL was not present in previously studied HfBSiCN films since they were either amorphous (high N, low Si) or partially recrystallized with presence of N containing phases BN, HfN, Si 3 N 4 . Oxidation of these phases occurred almost simultaneously at the same film depth resulting in the common to all top OL morphology involving HfO 2 nanoparticles dispersed in a SiO x -based matrix.
Thus, the oxidation mechanism in the present two films is composed of two stages. The first stage in the Hf 6 B 10 Si 31 C 2 N 50 film involves oxidation at the AL/TL interface of individual elements out of the Hf-rich short range ordered clusters forming small t-HfO 2 structures simultaneously with the formation of Si 3 N 4 nanocrystalline structures out of light elements motifs, Figure 4b. In the Hf 7 B 10 Si 32 C 2 N 44 film, the RL is composed of HfC x N 1−x and Si 3 N 4 nanocrystalline phases and oxidation of HfC x N 1−x to HfO 2 takes place gradually as the top RL section is approached. This gradual oxidation of the HfN phase produces a gradient microstructure in terms of HfO 2 content and as a result a distinct RL/TL interface is not observed, Figure 8. The onset of the TL can be considered at the point when most of the HfN phase has been oxidized and the resulting microstructure is composed of Si 3 N 4 and t-HfO 2 nanostructure network, Figure 11a, as the one in the TL of the Hf 6 B 10 Si 31 C 2 N 50 film, Figure 6.
The second stage of oxidation has a common mechanism in both films and occurs at the TL/OL interface. Oxidation of crystalline Si 3 N 4 to β-SiO 2 followed by its transformation to amorphous SiO 2 takes place in parallel with the coalescence of small t-HfO 2 and growth to large spherical m-/o-HfO 2 nano particles. We were able to capture the details of this second oxidation process while in progress for both films as shown in Figures 4a and 12. The high oxidation resistance of both Hf 6 B 10 Si 31 C 2 N 50 and Hf 7 B 10 Si 32 C 2 N 44 films is attributed to a similar microstructure that develops at the interfaces regardless of the different microstructures in the bottom layer.
Besides the above two oxidation stages, high density of low thermal conductivity (0.49-0.95 W· m −1 ·K −1 ) HfO 2 nanoparticles surrounded by high density SiO 2 quartz boundaries in OL form also an effective oxygen and thermal diffusion barrier at the interface resulting in high temperature oxidation resistance [27].

Conclusions
The Hf 6 B 10 Si 31 C 2 N 50 and Hf 7 B 10 Si 32 C 2 N 44 films annealed up to 1500 • C in air developed a three-layered microstructure. Both Hf-B-Si-C-N films formed a fully oxidized layer at the top surface with a nanocomposite structure of m-HfO 2 and/or o-HfO 2 embedded in an amorphous SiO x -based matrix. The bottom layer remained amorphous in the Hf 6 B 10 Si 31 C 2 N 50 film and it was partially recrystallized in the Hf 7 B 10 Si 32 C 2 N 44 film. The latter recrystallized layer was composed of a uniformly distributed h-Si 3 N 4 major phase along with HfC x N 1−x . Some t-HfO 2 appeared in the top section of this layer resulting from the oxidation of the HfC x N 1−x phase. This process has taking place gradually along the thickness of the layer with the amount of t-HfO 2 increasing as the top layer section was approached. Between the above two layers, both films exhibited a transition layer composed of a t-HfO 2 nanostructure network surrounded by Si 3 N 4 .
The fully oxidized/transition layer interface of both annealed Hf 6 B 10 Si 31 C 2 N 50 and Hf 7 B 10 Si 32 C 2 N 44 films was characterized by densely packed small HfO 2 , Si 3 N 4 and quartz SiO 2 nanostructures that can act as a major barrier for oxygen and thermal diffusion. The growth of HfO 2 nanoparticles at the interface of the oxide/transition layer is accomplished by merging the small HfO 2 grains into large HfO 2 particles via the oxidation of Si 3 N 4 to quartz SiO 2 and its transformation to amorphous α-SiO 2 . In addition, the transition layer composed of Si 3 N 4 and HfO 2 nanostructures could provide a second barrier for oxygen and thermal diffusion.