1. Introduction
New tool materials or coatings are needed for special applications, particularly in machining of difficult-to-machine materials [
1]. Coatings should be characterized by hardness, strength, and chemical inertness. Such properties are exhibited by boride films [
2,
3,
4,
5,
6,
7]. There are numerous compounds with boron that have good wear resistance, such as ZrB
2, TiB
2, BN, and Ti(B,N) [
8]. In addition, tungsten borides and ternary transition (TM) metal-diborides possess very high application potential. The ternary TM-diboride coating films showing exceptionally high phase stability and mechanical properties, even at high temperature or even after exposition to high temperatures [
2]. Addition of TM such as Ta or Ti cause that coatings possess fracture toughness
kIC (~3.5 MPa√m) higher than well-known coatings such us (Ti,Al)N, CrN/TiN, Ti–Si–N, TiN, and TiB
2 and at the same time they are super hard
H > 40 GPa [
2]. So far, there has been no research on the behavior of such layers under operating conditions. In Ref. [
9] a series of borides (CrB
2, Mo
2B
5 and WB) were applied as protective coatings on cutting tools. Turning tests carried out on titanium in dry conditions showed that WB crystalline coating was resistant to abrasive wear. Similar properties were observed by Chrzanowska-Giżynska et al. [
10,
11], who investigated the friction coefficient and wear resistance of αWB coatings on flat stainless steel substrates in scratch tests. αWB films deposited by radio frequency magnetron sputtering (RF MS) were characterized by good adhesion. The 1 μm films tested in the scratch test failed at 200 mN due to cracking; however, further increase in the load did not cause spallation of the coating [
10]. This study also indicated high wear resistance of 2.7 × 10
−6 mm
3·N
−1·m
−1 of such coatings.
Jiang et al. [
12] showed that direct current magnetron sputtered (DC MS) αWB
2 coatings with a metastable AlB
2-type structure possessed a nanocomposite structure exhibiting superhardness of about 43.2 ± 5 GPa. In this case, the steady-state friction coefficient measured during sliding was 0.23. Additionally, the wear rate of the films was 6.5 × 10
−6 mm
3·N
−1·m
−1, indicating that AlB
2-type WB
2 coatings have good potential application as superhard and low-wear coatings. Moreover, TiB
2-coated tools compared to uncoated tools showed better wear resistance in the case of machining titanium alloy in rough turning conditions. However, it was stated that the efficiency of the coatings strongly depended on the cutting conditions [
13].
Mixing tungsten and titanium borides causes other special features. For example, alloying tungsten boride coatings with titanium improved mechanical properties [
4,
14,
15,
16,
17,
18]. Euchner et al. [
14] showed that films of solid solutions of Ti
1−xW
xB
2−z deposited by magnetron sputtering (MS) over the whole composition range were crystallized in the AlB
2 structure type. The obtained coatings exhibited good thermal stability and high hardness with a maximum value of 40 GPa under 25 mN load for Ti
0.67W
0.33B
2−z [
14]. However, as shown in Ref. [
19], nanocomposites can also be formed in regions with high tungsten content, which is optimal for achieving high strength. The maximum hardness (33 GPa under 70 mN load) was obtained for a nanocrystalline composite based on the phase (Ti,W)B
2 with Ti content of 10.2 at.%. A similar observation was made by Smolik et al. [
20,
21], who also studied the fracture toughness. Tungsten alloying of titanium diboride coatings shows that the addition of 10% of tungsten causes a more than seven-times increase in the fracture toughness
KIC, from
KIC(TiB₂) = 0.67 to
KIC(TiBW) = 4.98 MPa·m
1/2. At the same time, Ti–B–W films were characterized by greater hardness (
H = 38 GPa under 400 mN load) and similar surface roughness compared to TiB
2 coatings [
20]. In both cases [
14,
20], films were deposited by DC magnetron sputtering.
Chrzanowska-Gizynska et al. [
16] used a hybrid MS + pulsed laser deposition (PLD) method for doping of α-WB lattice with titanium. At a titanium content of 5.5 at.%, the hardness of titanium-doped coatings was 47–50 GPa under 2.5 mN load. Additionally, the titanium alloying resulted in a change in the α-WB structure to a mixed α-WB and AlB
2-type WB
2 structure. Such a high hardness was also reported by Moscicki et al. [
4]. In this work, the authors studied the impact of titanium content on the properties of thin films deposited by RF magnetron sputtering from W
1−xTi
xB
4.5 (where
x = 0–0.24) spark plasma sintered targets. The results of the study showed that the addition of titanium apparently changed the film structure from nanocrystalline columnar to amorphous, a very dense and compact structure, with the addition of TiB
2 phase [
4]. Deposited coatings with titanium content of 5.5 at.% were simultaneously hard (
H > 37.5 GPa under 7 mN load) and had high hardness to effective Young’s modulus ratio values (
H/
E* > 0.1) and elastic recovery (
We > 60%) appropriate for toughness and resistance to cracking materials [
22]. (W,Ti)B
2 coatings with AlB
2 structure type also exhibited enhanced tribological and corrosive properties [
4].
The described properties of (W,Ti)B2 and α-WB2 films indicate great potential for using them as protective coatings for cutting tools. So far, there has been no research on the behavior of such layers under operating conditions.
In this work, the above-mentioned borides (α-WB2, (W,Ti)B2) are applied as wear-resistant coatings to commercial WC–Co cutting inserts. Turning tests carried out on difficult-to-machine 304 stainless steel showed that the W–B film caused less wear compared to uncoated inserts. After cutting, test flank wear was examined. Such wear is often used to predict tool life and has a big influence on the accuracy of machining. The properties of boride films and the possibility of using boride coatings to protect WC–Co cutting tools for difficult-to-machine materials were studied.
2. Experimental Procedure
The sputtering target was made by spark plasma sintering (SPS) from boron powder (~625 mesh, 99.7% purity; Sigma Aldrich, St. Louis, MO, USA), tungsten (12 μm, 99.9% purity; Sigma Aldrich), and titanium (45 µm, 99.98% purity; Sigma Aldrich). For deposition of α-WB
2 film, powder was mixed and sintered with a 4.5:1 (B/W) ratio [
23]. In the case of the target with titanium alloy, the ratio of boron to transition metals was 4.5 and Ti/(Ti + W) = 0.16 [
4]. The first target consisted of two WB
2 and WB
4 phases. The second target was a nanocomposite of WB
4, WB
2, and TiB
2 phases. Detailed information about the manufacturing process and the chemical structure and mechanical properties of targets were described in Refs. [
23] and [
4], respectively. Based on our earlier studies on sputtering power and gas pressure [
11], substrate temperature [
10], and composition [
4], the optimal parameters of deposition were chosen. The one-inch diameter target was mounted in the water-cooled magnetron sputtering cathode (Kurt J. Lesker, Jefferson Hills, PA, USA) and positioned at a distance of 50 mm in front of the target. The RF sputtering power was 50 W and bias potential on the substrate was floating. The deposition process occurred in a vacuum chamber initially pumped to 0.02 Pa and then filled with argon to a working pressure of 0.9 Pa. The gas flow of argon was 19 mL/min. Prior to each deposition, the target was sputtered for 5 min in order to ensure a clean surface and stable sputtering conditions. Film was deposited for 120 min on flat, polished substrate cut from WC rod (1 mm thick, Ø = 15 mm diameter; Stjorsen Polska, Tychy, Poland) and on commercial WC cutting tools. Before deposition, all substrates were cleaned with subsequent rinses in acetone, alcohol, and deionized water. Both substrates were mounted on a rotating holder and heated to 540 °C.
The crystal structure of the coating was investigated by X-ray diffraction (XRD) using Bruker D8 Discover (Cu radiation, λ = 1.5418 Å, Coventry, UK). Film measurement was taken in 2θ scan mode, with the source fixed at 8° position, which avoided signal from the substrate while maintaining high intensity of the signal originating primarily from the coating.
The surface was investigated with scanning electron microscopy (SEM; JEOL JSM6010PLUS/LV, Tokyo, Japan). Energy dispersive X-ray spectroscopy (EDS; JEOL (EUROPE) SA, Warszawa, Poland) microanalysis was used to study the elemental distribution. During EDS measurement, accelerating voltage of 7 kV was used. Due to problems with boron measurement and subsequently determining its quantity in combination with other, especially heavy elements, before measurement, the EDS spectrometer was calibrated based on polished W2B5 commercial standard (Huizhou Tian Yi Rare Material Co, Guang Dong, China, 99.9% purity).
Vickers microhardness tests were performed under 20 g load on a Hanneman tester (Zeiss, Jena, Germany). Five imprints were done on each tested sample. To evaluate film hardness, the substrate hardness was taken into account.
Surface roughness was measured on a Talysurf stylus scanning profilometer (Taylor Hobsson, Leicester, UK) according to ISO standards. The tip radius was 2 μm, traverse length according to ISO 4288 [
24] was 1.25 mm, and cut-off was 0.25 mm. A standardized phase-correct Gaussian high-pass filter (ISO 11562 [
25]) was applied. The profiles and surface measurements were done before and after the deposition process.
Wear tests were done on a Skoda–Savin tester (Skoda Works, Plzeň, Czech Republic) in semi-dry conditions. The test allows determination of wear at the semi-dry sliding friction. Determination of wear was made during rotational movement of the counter-sample made of WC–Co alloy sliding on a stationary sample while wetting the surface with cooling fluid. The following wear processes may occur: abrasion, chipping, surface plastic deformation, and adhesion.
Figure 1 shows the scheme of the wear test.
The roller was pressed against the tested surface at constant load of 15 kg. Relative motion speed was 1.5 m/s. Preliminary tests showed that after 500 rotations, the depth of the worn area was less than 1 μm, so each test was stopped after 500 rotations, corresponding to the length of the friction span of 170 m. The coolant, 0.05% K2CrO4 solution in distilled water, was applied with flow efficiency of 1 L/min. Five tests were performed on each tested surface. After the tests, surface topography was studied by laser confocal microscopy (Keyence VK-X100, Osaka, Japan) and the volume of worn material was determined.
The cutting test was performed on a Haas CNC TL1 lathe (Haas Automation, Oxnard, CA, USA). The turning process was performed on AISI 304 stainless steel. The constant cutting length was 70 mm and the following process parameters were applied: cutting speed
vc = 80 m/min, cutting depth
ap = 1.5 mm, and feed rate
p = 0.09 mm/rev. Two commercially produced cemented carbide (WC–Co) cutting tool inserts, supplied by Sandvik Polska Sp. z o.o. (Warsaw, Poland), were tested with and without deposited coatings. Nominal composition of inserts was 94% WC and 6% Co (H13A). Five tests were performed on each insert. The insert was rotated about 70° to perform the next test. The cutting tool with marked test location is illustrated in
Figure 2. After the cutting tests, the wear of inserts was observed by laser confocal microscopy and flank wear
VB was determined.