Adhesion Improvement in Diamond Films on Stainless Steel Under Scratch Conditions via Cr/CrSiVN Interlayers

Abstract

Diamond films deposited on stainless steel using a Cr/CrSiN interlayer exhibit excellent adhesion under static conditions but poor adhesion under scratch conditions. We developed a new interlayer Cr/CrSiVN and investigated the influence of different vanadium (V) contents on the adhesion. The adhesion of diamond films under scratch conditions is significantly improved with V content increasing from 0 to 4.1 at.%. Specifically, the critical load increases from 2.2 ± 0.8 N (0% V) to 21.2 ± 0.8 N (4.1% V), corresponding to a roughly 864% enhancement. High adhesion is ascribed to a certain thickness of carbide, the reduction in pores at the diamond/interlayer interface, and the enhanced hardness of the CrSiVN interlayer by solid solution strengthening. The formation rate of carbide is reduced due to a low carbon diffusion by the substitution of Cr atoms by V atoms in the interlayer, leading to a low carbonization rate of the interlayer and small pores at the interface. This work is of great significance to the promotion and practical application of diamond films in medical and food-processing equipment.

1. Introduction

Diamond exhibits many outstanding properties, including high hardness, good wear resistance, and excellent corrosion resistance [1]. Depositing a diamond film on stainless steel improves the wear and corrosion resistance of the product, thus prolonging its service life. The commonly used methods for preparing diamond films include hot filament chemical vapor deposition (HFCVD) and microwave plasma chemical vapor deposition (MPCVD). HFCVD is often used for depositing diamond films on stainless steel substrates due to its relatively simple setup, low preparation cost, and capability for large-area deposition. However, the direct deposition of diamond film on a stainless steel surface faces three main challenges. Firstly, metallic elements such as iron and nickel in stainless steel catalyze the formation of a graphite phase on the substrate surface [2]. Diamond nucleation and growth then occur on this graphite layer, but due to the weak interfacial bonding, the diamond film tends to detach from the substrate during cooling [3]. Secondly, the high diffusivity of carbon in stainless steel makes it difficult to reach the critical carbon concentration required for nucleation, leading to a low deposition rate. Thirdly, the significant mismatch in the coefficient of thermal expansion (CTE) between stainless steel and diamond leads to high thermal stress in the diamond film during cooling. This results in the diamond film wrinkling, cracking, or delaminating [4,5].

In order to successfully deposit diamond film on stainless steel, researchers generally employ interlayers. So far, a wide variety of materials have been selected as interlayers, including metal interlayers such as Cr [6], Ti [7], Al [8], Mo [9], and binary nitride interlayers such as Mo-W [10,11], CrN [12], AlN [13], and WN [14]. Among these interlayers, the binary CrN interlayer has attracted considerable interest, because an adherent diamond film can be obtained by the formation of chromium carbon compounds on the interlayer which has a good adhesion strength between diamond and the interlayer. However, CrN exhibits relatively low hardness and easy carbonization, which adversely affects mechanical properties. So, we developed a novel ternary nitride interlayer Cr/CrSiN , which has a higher hardness and the carbonization extent can be tuned through the content of Si (or N) [15]. The diamond film shows good adhesion in indentation tests (i.e., under static conditions) when deposited on stainless steel with the Cr/CrSiN interlayer. However, the adhesion performance is poor in scratch tests with a critical load of only several newtons. This affects its practical applications on stainless steel mechanical components subject to relative motion in industries such as medical or food processing. Therefore, it is necessary to improve the adhesion of diamond film under scratch conditions.

In the process of depositing diamond films using an interlayer on stainless steel, the adhesion of the diamond film under scratch conditions is influenced by many factors, such as the stress and diamond content of the diamond film, the microstructure of the diamond/interlayer interface (i.e., the type and thickness of the carbide layer formed by carburization of the interlayer in the HFCVD process, as well as the number of pores at the diamond/interlayer interface), and the hardness of the interlayer beneath the diamond film. The thermal stress in diamond films on stainless steel is typically 6–8 GPa [16]. It is almost impossible to reduce it due to the large CTE difference between the diamond film and the stainless steel, as well as the high deposition temperature. The CrSiN interlayer undergoes carburization and forms chromium carbides during HFCVD deposition of diamond film [15]. Introducing V into the CrSiN interlayer may change the microstructure of the diamond/interlayer interface due to the different interaction between V atoms and C atoms in the deposition of diamond film, leading to improved adhesion. Additionally, it can enhance the hardness of the interlayer due to solid solution strengthening [17], contributing to improved adhesion of the diamond film. Therefore, a highly adherent diamond film on stainless steel under scratch conditions may be obtained by using a CrSiN interlayer with V addition.

However, no relevant studies on the deposition of diamond films on stainless steel using this interlayer have been reported to date. Therefore, we deposited diamond films on stainless steel using CrSiVN interlayers with different V contents and investigated the effect of the interlayer on the microstructure of the carbide formed via carbonization of the interlayer and the adhesion of diamond films.

2. Materials and Methods

2.1. Preparation of the Cr/CrSiVN Interlayer

Annealed 3Cr13 stainless steel substrates with a size of 10 × 10 × 3 mm³ were ground sequentially with abrasive papers of 320, 600, 1000, 1500 and 2000 mesh. The samples were then ultrasonically cleaned in acetone for 20 min to remove surface impurities. Subsequently, they were placed into the magnetron sputtering apparatus (JGP-450, Shenyang Sky Technology Development Co., Ltd., Shenyang, China) to deposit the Cr/CrSiVN interlayer using high-purity (99.99%) Cr, Si, and V targets, each with a diameter of 60 mm. A Cr layer (~0.8 μm) and CrSiVN layer (~2 μm) was deposited on the stainless steel substrate. The specific deposition parameters are listed in

Table 1. Deposition parameters of Cr/CrSiVN interlayer.

	Cr Layer	CrSiVN Layer
Flux of N2/Ar (sccm)	0/20	5/20
Base pressure (Pa)	3 × 10−3
Working pressure (Pa)	0.8
Substrate temperature (°C)	300
Bias voltage (V)	−100
Power of Cr (W)	200
Power of Si (W)	0	60
Power of V (W)	0	0/30/60/90/120
Deposition time (min)	20	60
Rotation rate of substrate (r/min)	4

. The elemental composition of the Cr/CrSiVN interlayers was analyzed by EDS. The V content in the CrSiVN layers was 0, 1.8, 4.1, 6.8, and 9.7 at.% at V target sputtering powers of 0, 30, 60, 90, and 120 W, respectively. The corresponding samples with the interlayer were labeled as V-0, V-1.8, V-4.1, V-6.8, and V-9.7, respectively.

2.2. Preparation of Diamond Films

2.2.1. Pretreatment of the Sample

In order to improve the nucleation rate of diamond, the sample coated with the Cr/CrSiVN interlayer was subjected to ultrasonic seeding for 30 min in a diamond suspension containing acetone (30 mL), diamond powder (0.3 g, 250 μm), and Al₂O₃ (0.2 g, 63 μm). The addition of Al₂O₃ to the diamond suspension was intended to generate additional defects on the interlayer surface, thereby increasing the density of nucleation sites and ultimately improving the diamond nucleation rate. Subsequently, the samples were ultrasonically cleaned in acetone for 5 min to remove any residual diamond suspension from the surface.

2.2.2. Diamond Film Deposition

Diamond films were deposited on the pretreated samples using an HFCVD apparatus (JUHFCVD001, Shanghai Jiao Tong University, Shanghai, China). Six tantalum (Ta) filaments, each 18 mm in length, were used as the hot filaments. Acetone, serving as the carbon source, was delivered into the reactor using H₂ as the carrier gas. The diamond film was deposited using a two-step dropped power process [5]: diamond firstly nucleated at high power to improve the adhesion through the formation of high diamond content at the interface, then the power is quickly reduced to the low power before the formation of continuous diamond film in order to reduce the stress and the film grows at low power. The deposition parameters for the diamond film are listed in

Table 2. Deposition parameters of diamond film.

	First Step	Second Step
Deposition power (W)	1800	1600
Deposition time (min)	20	50
Substrate temperature (°C)	830	750
Bias current (A)	4
Working pressure (kPa)	1.38
Flux of H2 (sccm)	200
Flux of acetone (sccm)	80
Distance between substrate and wire (mm)	18
Rotation rate of substrate (r/min)	20

. Samples coated with diamond films using the Cr/CrSiVN interlayer of 0, 1.8, 4.1, 6.8, and 9.7 at.% V content were denoted as V-0-D, V-1.8-D, V-4.1-D, V-6.8-D, and V-9.7-D, respectively.

2.3. Characterization

Surface and cross-sectional morphologies of the films were observed using field emission scanning electron microscopy (FESEM, FEI Nova Nano, Golden, CO, USA). The elemental compositions of the sample were analyzed via energy-dispersive X-ray spectroscopy (EDS, Oxford, UK) at an accelerating voltage of 15 kV. Phase analysis was characterized via grazing incidence X-ray diffraction (GIXRD, Rigaku Smart Lab 9 kW, Tokyo, Japan) at a grazing incidence angle of 2°. Cross-sectional specimens were prepared following standard metallographic procedures, as described in our earlier publication [15]. The chemical state of vanadium (V) atoms in the delaminated region of the film was characterized by X-ray photoelectron spectroscopy (XPS) using a Kratos Axis Ultra DLD spectrometer (Kratos Analytical, Shimadzu, Japan) equipped with a monochromated Al Kɑ X-ray source (hν = 1486.6 eV). The X-ray source was operated at 16 kV and 3 mA, with an analysis area of 300 × 700 μm². High-resolution spectra were acquired using a pass energy of 80 eV, an energy step size of 50 meV, and 16 scan cycles, with charge compensation provided by a low-energy electron flood gun. Spectral deconvolution was performed using the XPS PEAK 4.1 software. The binding energies were calibrated with respect to the C–(C, H) component (BE = 284.8 eV) of the C1s peak, and a Shirley background subtraction was used. Spectral deconvolution was carried out with a mixed Gaussian–Lorentzian function (80% Gaussian, 20% Lorentzian). The diamond content and residual stress in the films were evaluated by micro-Raman spectroscopy (Renishaw inVia Reflex, UK) using a 514 nm laser with a spot size of approximately 2 µm. The adhesion of the films was assessed with a Rockwell C hardness tester (HR-150A, Shanghai Hesheng Instrument Co., Ltd., Shanghai, China) according to the VDI 3198 standard [18]. Each sample was tested three times under a load of 1470 N with a conical diamond indenter (120° cone angle, 0.2 mm top radius). The hardness of the films was measured using a digital microhardness tester (HX-1000TM, Shanghai Lidun Instrumentation Testing Technology Co., Ltd., Shanghai, China) with a load of 0.2 kgf. Each sample was tested three times. Scratch tests were performed on a scratch tester (WS-2005, Lanzhou ZKKH instrument Co., Ltd., Lanzhou, China) using a diamond stylus with a 0.2 mm top radius. The test was performed under the following conditions: maximum load of 40 N, scratch length of 3 mm, and loading rate of 20 N/min. Each sample was tested three times. The critical adhesion strength was determined as the load at which the diamond film first showed spallation along the scratch track (L_c2) [19,20].

3. Results

3.1. Microstructure of the Diamond Film

Figure 1 presents SEM images of carbon films deposited on stainless steel with Cr/CrSiVN interlayers with different V contents. As shown, the carbon films are all continuous and dense for samples with interlayers having vanadium contents ranging from 0 to 6.8 at.%. The grains of the films are very fine, typically in the range of 50–100 nm, in spite of a relatively larger size for samples V-1.8-D and V-6.8-D. Notably, the film for sample at 9.7 at.% V delaminated during cooling (see Figure 1e).

Figure 2 displays Raman spectra of diamond films deposited on stainless steel using Cr/CrSiVN interlayers with different V contents. Each Raman spectrum is deconvoluted into six Gaussian peaks by the least-squares method [21]. The peak at ~1332 cm⁻¹ is assigned to the F_2g vibrational mode of diamond [22,23]. The peaks at ~1350 cm⁻¹ and ~1580 cm⁻¹ are ascribed to the D band and G band of sp² graphitic phases, respectively [24]. The peaks at ~1140 cm⁻¹ and ~1470 cm⁻¹ are attributed to the C-C/C=C and the C=C stretching vibrational modes of trans-polyacetylene (TPA) chains, respectively [24]. The peak at ~1200 cm⁻¹ corresponds to the E_2g vibrational mode of amorphous sp³ carbon [23]. The fitting results show that the diamond peak positions are 1344.5, 1343.3, 1343.6, and 1344.2 cm⁻¹ for samples V-0-D, V-1.8-D, V-4.1-D, and V-6.8-D, respectively, indicating that the diamond films on the samples exhibit compressive residual stress. The stress in the diamond film is calculated by the following formula [21]:

$$\sigma =-0.567({\mathsf{\nu }}_{\mathrm{d}}-{\mathsf{\nu }}_0)$$

(1)

where σ is the diamond stress, negative sign represents compressive stress, ${\mathsf{\nu }}_{\mathrm{d}}$ and ${\mathsf{\nu }}_0$ are the position of the diamond characteristic peak and unstressed diamond peak (1332 cm⁻¹), respectively. The calculated values are listed in

Table 3. Diamond peak position, residual stress, and diamond content of diamond film prepared on stainless steel using Cr/CrSiVN interlayers with different V contents.

Sample	Diamond Peak Position (cm−1)	Residual Stress (GPa)	Diamond Content (%)
V-0-D	1344.5	−7.1	35.0
V-1.8-D	1343.3	−6.4	33.2
V-4.1-D	1343.6	−6.6	30.8
V-6.8-D	1344.2	−6.9	35.7

. As shown, the stress in the diamond films is in the range of 6.4 to 7.1 GPa. The high stress is primarily attributed to thermal stress resulting from the large CTE difference between the steel substrate and the diamond film [25,26].

Moreover, the diamond content is calculated by formula [21]:

$${\mathrm{C}}_{\mathrm{D}}={\displaystyle \frac{{\mathrm{I}}_{\mathrm{D}}}{{\mathrm{I}}_{\mathrm{D}}+(\frac{{\mathrm{I}}_{\mathrm{C}}}{233})}}\times 100\%$$

(2)

where ${\mathrm{C}}_{\mathrm{D}}$ is the diamond content, ${\mathrm{I}}_{\mathrm{D}}$ is the peak area of the diamond peak, and ${\mathrm{I}}_{\mathrm{C}}$ is the sum of the peak area except for the diamond peak. The calculated diamond contents are also shown in Table 3. It is seen that the four samples exhibit comparable diamond contents of approximately 35%. In short, the V content in the interlayer does not significantly affect either the residual stress or the diamond content of the diamond film deposited on stainless steel.

3.2. Adhesion of the Diamond Film

The adhesion of diamond films deposited on stainless steel with chromium nitride-based interlayer is generally excellent under static indentation testing but poor under scratch testing [27]. We used Rockwell C indentation testing and scratch testing to evaluate the adhesion of diamond films on stainless steel under static and scratch conditions, respectively. Figure 3 shows SEM images of the indentation imprint on all diamond films. No delamination around the imprint is observed for samples V-0-D, V-1.8-D, and V-4.1-D under a load of 150 kgf, indicating good adhesion of the diamond film at a V content of 0–4.1 at.% under static conditions. According to the VDI 3198 standard, the adhesion of these diamond films corresponds to grade HF1 [18]. However, extensive delamination of the diamond film was observed around the indentation crater for sample V-6.8-D (see the inset in Figure 3d), suggesting poor adhesion of the diamond film at a V content of 6.8 at.%. Considering that the diamond film delaminated on the sample V-9.7-D as described in Section 3.1, it is believed that the adhesion of diamond film under static conditions becomes poor at a V content larger than or equal to 6.8 at.%.

Figure 4 shows SEM images of the scratches on all diamond films. The critical load (L_c2) of the diamond film firstly rises from 2.2 ± 0.8 to 21.2 ± 0.8 N as the V content increases from 0 to 4.1 at.%, then it decreases to 17.6 ± 0.5 N as the V content reaches 6.8 at.%, suggesting that scratch adhesion can be improved by adding an appropriate amount of V to the interlayer. In addition, EDS analysis was performed on the delaminated areas (see white square in Figure 4). As shown in

Table 4. Composition of delaminated regions of samples with different V contents after the scratch test.

Sample	Element Content (at.%)
	C	Cr	Si	V	N	Fe
V-0-D	54.1	41.4	1.2	0.0	0.0	1.2
V-1.8-D	49.7	46.8	0.7	1.6	0.0	1.1
V-4.1-D	46.8	48.1	0.9	2.9	0.0	0.9
V-6.8-D	48.2	45.7	0.5	3.8	0.0	1.8

, there is a large amount of Cr and C elements and a small amount of Fe, Si, and V elements at the delamination site for all samples. It is believed that the failure occurs at the diamond/Cr-C interface in the scratch test considering that the stress concentration of the diamond film and that of the Cr-C interlayer are very different.

3.3. Analysis of the Reasons for Adhesion Improvement

The scratch adhesion of diamond film on stainless steel is influenced by multiple factors, including the residual stress and diamond content of the diamond film, the microstructure of the diamond/carbide layer interface (such as the type and thickness of the carbide layer, as well as the presence of pores at the interface) , and the hardness of the interlayer.

High residual stress typically leads to poor adhesion [28]. Films with a high diamond content exhibit high hardness, which enhances its resistance to plastic deformation and consequently improves adhesion in scratch tests [29]. As mentioned before, the stresses in diamond films with different V contents are comparable, and the diamond content is also similar for samples. Therefore, the improvement in adhesion with V addition cannot be ascribed to variations in stress or the diamond content of the film.

The type of carbide also affects the adhesion of the diamond film on stainless steel. A higher proportion of Cr₃C₂ generally leads to better adhesion, because the adhesion strength between diamond and Cr₃C₂ is greater than that between diamond and Cr₇C₃ [30]. As shown in Figure 5a, both Cr₃C₂ and Cr₇C₃ carbide phases exist in all samples. The ratio of the Cr₃C₂ peak area to the Cr₇C₃ peak area in the GIXRD pattern was calculated, representing the relative volumetric content of these chromium carbides in each sample. It is seen from

Table 5. Peak area of chromium carbide of Cr₃C₂ and Cr₇C₃ in GIXRD.

Sample	Peak Area of Cr3C2 (S1)	Peak Area of Cr7C3 (S2)	S1/(S1 + S2)
V-0-D	38,178	8632	81.6%
V-1.8-D	3550	2739	56.4%
V-4.1-D	4605	5077	47.6%
V-6.8-D	1793	6302	22.1%

that the Cr₃C₂ content decreases as the V content in the interlayer increases. The V-0-D sample exhibits the highest content of Cr₃C₂ carbide phases (81.6 vol%) without the addition of V, while sample V-6.8-D has the lowest (22.1 vol%). Therefore, adhesion would be expected to decrease with increasing V content from 0 to 4.1 at.%. However, this is not the case, implying that other factors must be responsible.

The existence of vanadium carbides such as VC, V₄C₃, and V₆C₅ also can enhance adhesion strength [31,32]. However, XRD cannot detect carbide phases present at contents below about 5%, so their presence remains uncertain. Given that XPS is capable of identifying small amounts of carbides, we employed it to detect the existence of the carbide in the delaminated region of the sample. The V-9.7-D sample was chosen given that more vanadium carbide is likely to form when V content is high. Figure 5b shows the XPS spectra of the substances at the delaminated region for sample V-9.7-D. No V-C peak is observed at 512.6 eV, indicating that V atoms in the interlayer did not react with carbon atoms during the CVD process. The V2p_3/2 peaks at 514.1 eV and 517.3 eV correspond to V-N and V-O bonds, respectively. The V-O peak is considered to be related to atmospheric contamination. The absence of vanadium carbide and the predominant formation of chromium carbide can be ascribed to the lower free energy of formation of chromium carbides (Cr₇C₃: −153.6 kJ/mol, Cr₃C₂: −791.0 kJ/mol) compared to vanadium carbides (VC_0.73: −97.0 kJ/mol, VC: −102.1 kJ/mol, V₂C: −146.4 kJ/mol) [33].

The thickness of the carbide layer also plays a role in the adhesion. Only a carbide layer above a certain critical thickness can ensure high adhesion. When the carbide layer is thinner than this critical value, the diamond film exhibits poor adhesion because it fails to provide sufficient bonding strength at the interface [34]. Figure 6 shows EDS line scanning for the samples with different V contents. It is observed that the carbide layer thicknesses are 0.9, 0.8, 0.7, and 0.6 μm for samples V-0-D, V-1.8-D, V-4.1-D, and V-6.8-D, respectively, indicating that the thickness decreases with increasing V content. The samples V-0-D, V-1.8-D, and V-4.1-D show good adhesion, whereas the adhesion of sample V-6.8-D becomes poor. Sufficient diamond film adhesion is only available when the carbonized interlayer carbide reaches a critical thickness. So, it is considered that the diamond film on the sample has lower adhesion due to a thinner carbide layer at a higher V content. Here, the reduced thickness of the carbide layer can be attributed to the slowing of carbon diffusion rate by the incorporation of V atoms, as mentioned above.

Additionally, it is reported that the defects or pores at the diamond/interlayer interface deteriorate the adhesion under scratch conditions [27]. Figure 7 shows cross-sectional images of the samples. It is observed that the carbon films’ thicknesses for samples V-0-D and V-1.8-D are all 2.1 μm, while those of samples V-4.1-D and V-6.8-D are slightly larger, at 2.2 μm. This suggests that the carbon film thickness is almost the same for all samples. The diamond/interlayer interface is flat in each sample. As shown in Figure 7, many pores with a size of 20–50 nm are visible at the interface for sample V-0-D. In contrast, the number of pores becomes small and the size of the pore remains nearly unchanged when a vanadium-incorporated interlayer is adopted. Three SEM fields were randomly selected for each sample, corresponding to a total interface length of ~30 μm. The interfacial porosity at the diamond/CrSiVN interface was determined by counting visible pores and normalized to the number of pores per 10 μm of interface length. The calculated values for samples V-0-D, V-1.8-D, V-4.1-D and V-6.8-D are 5, 3, 1, 1 pores/10 μm, respectively. This indicates the volume of pores at the diamond/interlayer interface decreases as more V is incorporated into the interlayer. The improved adhesion of the diamond films can also be ascribed to the reduction in porosity at the diamond/interlayer interface.

The hardness of the interlayer also influences the adhesion of the diamond film [35,36]. A high-hardness interlayer beneath the diamond film reduces the difference in plastic deformation between the diamond film and the interlayer under load, leading to a higher critical failure force during scratching. Figure 8 shows the hardness of the sample with CrSiVN interlayer with different V contents. It rises progressively from 458.1 ± 35.9 to 963.2 ± 29.6 HV 0.2 as the V content increases from 0 to 9.7 at.%. Therefore, it is believed that the improvement in diamond film adhesion under scratching conditions is partially attributed to the increase in the hardness of the interlayer.

In all, the enhanced adhesion of diamond film on the stainless steel using CrSiVN interlayer is caused by formation of a carbide layer with a thickness larger than a critical value, reduced porosity at the diamond/interlayer interface, and increased hardness of interlayer, rather than residual stress, diamond content, or carbide type.

In order to understand the effect of CrSiN interlayer with V incorporation on the thickness of carbide layer, the porosity at the interface, and the effect of V content on the interlayer hardness, we used GIXRD and SEM to characterize the phase composition and grain size of the interlayer with different V contents, respectively.

Figure 9 shows the GIXRD patterns of the interlayer with different V contents. It is observed that when V is not added to the interlayer, there are three peaks at 37.4°, 42.7°, and 67.5°, which correspond to crystal planes (110), (111), and (300) of Cr₂N from the JCPDF card (No. 35-0803). This suggests that the interlayer mainly consists of (Cr,Si)₂N phase, which is in agreement with our previous result [21]. The peaks attributed to crystal planes (002) and (300) all shift to a low angle with increasing V content from 1.8 to 6.8 at.%. This peak shift is attributed to the substitution of Cr atoms (radius: 128 pm) by larger V atoms (radius: 134 pm). The disappearance of the peak at 42.7° corresponding to crystal plane (111) after V incorporation is related to the grain orientation. It is considered that the interlayer is also mainly composed of (Cr,Si,V)₂N solid solution after V incorporation. Additionally, the full width at half maximum (FWHM) of the peak attributed to crystal planes (111) and (300) is very large for sample V-0, suggesting the poor crystallinity of (Cr,Si)₂N in the absence of V. Compared to the FWHM of the peak attributed to crystal plane (300) (0.757°) for sample V-0, the FWHM of the crystal plane (300) becomes small after V addition and is 0.470°, 0.515°, and 0.554° for V contents of 1.8, 4.1 and 6.8 at.%, respectively. This indicates that crystallinity of the interlayer is improved after V addition and lattice distortion intensifies with increasing V content. This is believed to be associated with the higher bond energy of the V-N bond (523 $\pm$ 38 kJ/mol) than that of the Cr-N bond (377.8 $\pm$ 18.8 kJ/mol) [37]. In all, the interlayer after V addition is mainly composed of (Cr,Si,V)₂N solid solution with better crystallinity.

As shown in Figure 10, samples V-0, V-1.8, and V-4.1 are composed of grains with a size ranging from 100 to 150 nm. In sample V-6.8, the grain size also ranges from 100 to 150 nm, although some large cluster-like particles are also observed. In summary, the grain size of the interlayers is similar at different V contents.

Thus, the reduction in carbide layer thickness and porosity at the interface with the increase in V content in the interlayer can be explained as follows. Generally, the formation of Cr–C compounds consists of three steps: adsorption of carbon atoms on the CrSiVN interlayer, cleavage of M–N (M = Cr, V, Si) bonds, and subsequent generation of chromium carbides via Cr–C bond generation. The melting point of VN is 2360 °C, whereas that of Cr₂N is only 1650 °C [38], which indicates that the V–N bond possesses higher bonding energy than the Cr–N bond. Doping V into CrSiN films causes V atoms to replace Cr lattice sites. Higher V doping concentrations boost the average M–N bonding energy, hindering M–N bond breakage and carbide formation, and resulting in a thinner carbide interlayer. It is reported that the carbide is porous when the extent of carburization is large; conversely, the carbide is dense under conditions of limited carbon incorporation [27]. Therefore, pores at the interface decrease due to the decrease in carbonization extent using the interlayer at high V content. In regard to increased hardness of Cr/CrSiVN interlayer, this is attributed to solid–solution strengthening by the substitution of Cr atoms by V atoms with larger radius.

3.4. A Simple Model

In summary, the effect of V content in the Cr/CrSiVN interlayer on the preparation of diamond films on stainless steel is summarized in Figure 11. As shown in the figure, with increasing V content in the interlayer, more V atoms dissolve into (Cr,Si)₂N grains. This reduces the diffusion of C atoms during CVD deposition, leading to carbon supersaturation on the surface, which promotes rapid diamond nucleation. And the carburization degree of the interlayer is reduced, resulting in a thinner carbide layer and fewer pores at the diamond/carbide interface. The reduced porosity at the interface decreases stress concentration, thereby improving the adhesion of the diamond film. Moreover, the solid solution of V atoms in (Cr,Si)₂N grains increases the hardness of the interlayer, which also enhances adhesion, as a harder interlayer provides better support for the diamond film, minimizing plastic deformation under load and reducing the risk of film delamination. As a result, the adhesion under scratching conditions increases with V content increasing from 0 to 4.1 at.%. The subsequent decrease in the adhesion is ascribed to the excessively thin carbide layer, because it cannot provide enough interfacial bonding.

4. Conclusions

In this work, Cr/CrSiVN interlayers with different V contents were utilized to deposit diamond films on stainless steel to improve adhesion under scratching condition. The conclusions are as follows: the interlayer effectively improves the adhesion of diamond films on stainless steel. The adhesion improves with V content of the interlayer increasing from 0 to 4.1 at.%, then follows a decrease at higher V content. The critical load achieves a maximum value of 21.2 ± 0.5 N at 4.1 at.%. The improvement in adhesion is attributed not to the residual stress or diamond content of the diamond film, nor to the type of underlying carbide, but to the following factors: (1) Optimized thickness of the carbide layer; (2) Reduced pore density at the diamond/carbide interface, which lowers stress concentration; (3) Enhanced hardness of the interlayer due to solid–solution strengthening. The carbide layer thickness decreases with increasing V content, and the minimum thickness required for high adhesion is 0.6–0.7 μm. The addition of V to the interlayer also reduces the diffusion rate of carbon atoms, leading to a thinner carbide layer and lower porosity at the interface. This study presents a novel interlayer that enables the deposition of adherent diamond films on stainless steel under scratch conditions, paving the way for potential applications in medical or food-processing equipment.