1. Introduction
Chemical vapor deposition (CVD) can be defined as deposition of a solid onto a heated surface caused by a vapor-phase chemical process. It belongs to the atomistic class of vapor-transfer processes, which means that the deposition species are atoms, molecules, or a combination of both [
1,
2,
3]. Therefore, CVD is a technology that relies on the production of a gaseous species carrying the coating material within a coating reactor or chamber. Alternatively, the gaseous species could be created outside of the coating reactor and delivered through a delivery system. These gaseous species can come into direct contact with the surface that needs to be coated. The reactor is kept at a high temperature, which can reach up to 2000 °C [
4,
5]. The CVD process can be used to create a wide range of coatings, from soft, ductile coatings to those with hard, ceramic-like characteristics. Coatings can have a hardness ranging from 150 to 3000 HV (0.1 kgf). Currently, CVD coatings are being utilized to prevent the severe attrition of components used in various industrial situations where corrosion, oxidation, or wear occurs [
4,
6,
7].
Generally, TiN and TiC coatings have been used on cutting tools for over three decades and continue to be important components of current tools. Titanium nitride and titanium carbide (TiN, TiC) are well-known for their strong adhesion, high melting point, chemical stability, lack of phase change, high hardness, and wear resistance. The use of TiN and TiC improves the thermal stability, oxidation behavior, and abrasive wear of the coating [
8,
9]. Consequently, applying TiN and TiC coatings on cutting tool materials increases features such as hot hardness, thermal shock, oxidation, chemical stability, toughness, and tribological characteristics [
10].
So, Ti (C, N) is commonly used as a protective coating in the metal-cutting industry because of the combination of tough TiN and hard TiC characteristics [
11,
12]. Numerous studies [
13,
14,
15,
16] have shown that Ti (C, N) coatings exhibit low surface roughness and high hardness due to their fine-crystallized surface. Based on previous studies, the hardness of TiC- and TiN-based coatings produced by different methods (PACVD, PVD, and self-propagating high-temperature synthesis (SHS)) is approximately 30 GPa, while their elastic modulus is around 230 GPa. The temperature applied during the production of the samples was below 1000 °C in all cases. Tribological characteristics are crucial for evaluating the performance of cutting tools, and multiple studies have shown that Ti (C, N) coatings exhibit a lower wear rate. Ti (C, N)-coated tools have been reported to last twice as long as uncoated tools [
17,
18,
19,
20,
21].
It is worth mentioning that TiN belongs to the interstitial compound group and crystallizes in the fcc B1-NaCl structure. TiN is stable throughout a wide composition range. Bulk materials have been observed with nitrogen to titanium ratios ranging from 0.6 to 1.16. Under-stoichiometric films (N/Ti < 1.0) have vacancies on the N-sublattice, while over-stoichiometric films (N/Ti > 1.0) have vacancies on the Ti-sublattice. TiC has almost the same crystal structure as TiN (B1-NaCl), but due to stronger covalent bonding, it has a higher hardness. Miscibility can occur between TiN and TiC, where nitrogen in the fcc structure can be freely replaced by carbon, forming a TiCxN1-x (0 ≤ x ≤ 1) solid solution. The steady variation in composition allows for the regular adjustment of coating properties [
22].
The strong mechanical properties of TiN make it highly effective for use in a variety of applications. Additionally, TiN’s attractive golden color allows for wear detection and serves as a marketing argument. Thermal CVD deposition of TiN on cemented carbide tools, in particular, is a well-established procedure using the precursors TiCl
4, N
2, and H
2. TiCl
4 and N
2 provide the elements which constitute the coatings, while H
2 acts as a reduction agent. Argon is used as a carrier gas and to produce a total flow rate high enough to achieve the required mass fluxes and deposition rates. These films are typically stoichiometric, with minor chlorine incorporations. The coatings have a columnar microstructure and excellent adherence to cemented carbide [
23,
24]. The application of surface coatings allows for the production of components with specific tribological properties, such as low coefficients of friction, high wear resistance, or both. While the surface coating provides tribological functions, the bulk material can be selected based on characteristics such as stiffness, strength, or cost. Therefore, coating tools and machine components is a highly effective technique for increasing the lifetime and productivity of parts [
25,
26,
27].
The tribological behavior of materials has been studied by many researchers using a variety of techniques, including ball-on-disc, ball-cratering, fretting, rubber-wheel abrasion, etc. A crucial factor to consider when choosing a tribological model test is the type of stresses that the investigated material systems will experience in industrial use. The aim of the present study is to investigate TiN/TiC-based coatings with steel and zirconium static counterparts. When low surface roughness of ceramic products is critical, such as in high-pressure pistons, selecting the right tool for precise size grinding is important. In such cases, the specimen is subjected to linearly alternating kinetic stresses during use and machining, which are best modeled using a pin-on-disc tribometer [
28,
29,
30]. A ball-on-disc approach has been used by numerous authors to explore the wear behavior and wear rate of TiN and TiC coatings. Among them, researchers investigated the effect of sliding speed on the wear rate of TiN and TiC coatings at different linear speeds ranging from 5 to 30 cm/s (sometimes reaching up to 200 cm/s) against an Al
2O
3 ceramic ball and steel ball at different loads from 2.5 N up to 25 N, comparing the effects of the TiN and TiC coating layers on the wear performance of the cermet (which is used as a cutting tool insert). The cermet is a composite material made from ceramic and metal that can combine beneficial characteristics of both metals and ceramics, such as the capacity for plastic deformation and high temperature resistance and hardness. A carbide or oxide, such as WC, is bound to the metal with nickel, molybdenum, and cobalt typically being the metals used [
31,
32,
33,
34].
Moreover, researchers investigated the wear behavior of TiN and TiC coatings at high sliding speeds of about 200 cm/s with a normal load of 10 N and sliding distance up to 678 m against an SiC ball. They also tested the wear behavior of TiN (N-rich) and TiC (C-rich) coatings with a sliding speed of 20 cm/s, a normal load of 2.5 N, and a sliding distance of about 1000 m, using a traditional ball-on-disc tribometer to compare the tribological behaviors of TiN and TiC coatings against steel and alumina balls at two distinct contact loads of about 1 N to 5 N, and three different sliding speeds of 0.1, 0.3, and 0.5 m/s.
Although the CVD deposition of TiN and TiC is widely known, most studies focus on the growth of the coatings, mechanical characteristics, or tribological behavior. However, only a few consider the investigation of the tribological behavior of the coatings against different materials and investigate the effects of these results on the coating morphology and the resultant mechanical properties [
35,
36,
37,
38].
Typically, sliding tests to determine the wear rate, wear behavior, and coefficient of friction of TiN and TiC coatings are generally performed using balls made of alumina or steel. However, there are no investigation studies on the wear behavior of the coatings using a zirconia (ZrO2) ball.
The goal of this study is to investigate the tribological properties of two-layer coatings (TiC and TiN) created with various manufacturing parameters. Both sample preparations used WC–Co cermet as the substrate. To compare the wear mechanisms, steel and zirconia counterparts were used. The tests aimed to demonstrate that, in many cases, zirconia is preferred over steel due to its higher hardness and better wear resistance. Studying the wear and friction properties of zirconium is crucial for the ceramic industry, especially in areas that require precise fit and low surface roughness, such as medical applications, pistons, and grinding equipment.
2. Materials and Methods
2.1. Materials
The coatings have been produced by thermal CVD in an industrial reactor (Büttner Ltd., Nagyatád, Hungary) using the manufacturing parameters shown in
Table 1. The process was performed using the precursors titanium–tetrachloride (TiCl
4), nitrogen (N
2), hydrogen (H
2) and argon (Ar), where WC–Co cermet (Voestalpine Eifeler GmbH, Düsseldorf, Germany) was used as a substrate. In the case of Sample A, a lower temperature and twice the pressure was applied compared to Sample B. The precursor gas flow rate (Standard Liters per Minute—SLM) was kept at nearly the same value, and the TiC layer was produced under identical conditions for both samples.
The microstructure of the coatings formed on identical substrates is shown in
Figure 1. The red-marked image shows the backscattered electron images taken from the cross-sectional polished samples, where the layer thickness was measured. On the surface of Sample A (
Figure 1a), TiN is present in a more crystallized form, while Sample B (
Figure 1b) typically shows rounded shapes. The sharp crystals seen in Sample A may play an important role in terms of tribology, as they can remove material from the surface of the counterpart to a much greater extent.
The thicknesses of the different layers are summarized in
Table 2. The values presented in the table represent the results of 5 parallel measurements. In both samples, the thickness of the TiC layer formed is nearly the same, but in the case of Sample A, the thickness of the top layer is twice as much as that of Sample B. Presumably, the applied pressure value is the reason why many sharp crystals are present on the surface, in greater thickness.
The elemental composition of the different coatings is summarized in
Table 3. The measurements were performed on cross-sectional samples. The composition of the titanium-based coatings is nearly stoichiometric, with the Ti:C and Ti:N ratios being approximately 1:1 in each case.
Figure 2 illustrates the phase composition of the substrate and the coatings formed on it. Characteristic reflections of tungsten carbide and cobalt can be seen on the substrate. In the case of Sample B, the reflections of the deposited coating also appear, but with a much lower intensity than in the case of Sample A. As was also visible in the scanning electron microscope images, the TiN coating is present in a better crystallized form on the surface of Sample A, with characteristic reflections of this being much more intense.
2.2. Methods
Morphology was examined using a FEI/Thermo Fisher Apreo S scanning electron microscope (SEM, Brno, Czech Republic). The SEM observations were conducted under high-vacuum conditions with an accelerating voltage of 20.0 kV. To obtain the best resolution for back-scattered electron imaging, the samples were washed in an ultrasonic bath using ethanol and acetone. To prepare the cross-section, the samples were embedded in epoxy resin (NXMET XF40) and polished. The elemental compositions of the samples were determined using an EDAX Octane Elect Plus Energy Dispersive X-ray Analyzer (Ametek GmbH, Wiesbaden, Germany). The accelerating voltage was 20 kV and the data collection time was 180 s.
The microhardness test was performed on both the coated and uncoated (reference) samples and characterized by the Vickers microhardness (4.903 N, Wolpert 402 MVD, Fritz Müller GmbH, München, Germany) with a 0.2 kgf loading force. The hardness was recorded as an average of five readings for each specimen.
The X-ray diffraction (XRD) analyses of the samples were performed using a Philips PW 3710 diffractometer (PANanalytical B.V., Almelo, Netherlandsmanufacturer, city, state (only for USA and Canada), country). The measurement parameters were as follows: CuK radiation (50 kV, 40 mA), a speed of 0.02 2θ/s, a range of 10 to 70 degrees 2θ, and a curved graphite monochromator. The X’Pert Data Collector software (version 2.0e) was used to collect XRD pattern data. The High Score Plus 5.0 software was used to identify phases and to perform quantitative phase analysis using the Rietveld method. The crystalline phases were identified by comparing the XRD patterns with the 2021 Powder Diffraction Files (PDF-2 2021) from the International Centre of Diffraction Data (ICDD).
The tribological behavior was examined through dry-sliding experiments using an TRB
3 type tribometer (Anton Paar GmbH, Buchs, Switzerland) in a ball-on-disc arrangement. The pin-on-disc tests were carried out in accordance with ASTM G99 [
38] and ASTM G133 [
39] standards. The coatings were tested against steel (Anton Paar GmbH, Buchs, Switzerland) 100Cr6 steel ball) and zirconia (Fritsch GmbH, Idar-Oberstein, Germany, zirconium oxide grinding ball) balls with diameters of 6 mm and 5 mm, respectively, at 25 °C in ambient air with a relative humidity of 46 ± 3%. The maximum sliding speed and normal load were kept constant at 15.71 cm/s and 20 N, respectively. The coefficient of friction in relation to sliding distance was recorded during the tests. Then, the wear track was cleaned with alcohol to remove loose wear debris, and the wear track was measured using a profilometer. To compare the wear resistance of the coatings, the wear volume (
VP, mm
3) values were calculated according to the standards above:
where
h is the height of the removed material from the ball (mm), and
D is the diameter of the wear scar on the ball. The height of the material removed can be calculated as follows:
where
R is the original ball radius (mm).
A modified scratch test was used to investigate the adhesion of the coatings. During the examination, an TRB3 type tribometer (Anton Paar GmbH, Buchs, Switzerland) was used with a total stroke length of 10 mm and a constant normal force of 5 N. The diamond indentation marks on the surface were evaluated using a scanning electron microscope. Additionally, the surface roughness parameters were determined using a Surtronic S128 (Taylor Hobson GmbH, Leicester, England). A thermal camera (Trotec GmbH, Heinsberg, Germany) with model Trotec XC300 was used to investigate the change in temperature during the tribology test against steel and zirconia balls.