ICMCTF2012 Session B4-3: Properties and Characterization of Hard Coatings and Surfaces

Ternary nitrides find widespread application as hard protective coating on cutting tools and as corrosion and wear resistant coatings on mechanical components. Concerning to this, ternary nitride such (Cr_1‑xAl_x)N deposited via PVD (Physical Vapour Deposition) show outstanding tribological, mechanical and chemical properties. The combination of these properties makes (Cr_1-xAl_x)N coatings suitable for many applications. For an effective protection of coated parts a uniform layer of coating material is also required. In this regard, the HPPMS (high power pulse magnetron sputtering) technology offers possibilities to improve coating thickness uniformity as well as to enhance mechanical properties. The present work deals with the investigation of the influence of process parameters on the (Cr_1-xAl_x)N coating properties deposited at 200 °C via combination of DC-MSIP and HPPMS technology. The aluminum content of the (Cr_1-xAl_x)N was varied between 22 at‑% and 88 at‑%. Further, the deposition pressure was varied in the range of 475 mPa and 525 mPa. Subsequently, the bias voltage was varied from 0 up to -200 V in order to analyze the coating thickness distribution on surfaces parallel and perpendicular to cathode. Mechanical properties, morphology and phase composition were analyzed by means of Nanoindentation, SEM (Scanning Electron Microscopy) and XRD (X‑Ray diffraction) measurements. The results show that the coating with 31 at-% Al displays higher hardness (20 GPa) and more favorable Young’s modulus (370 GPa) compared to the other coatings. The increase of the deposition pressure from 475 mPa to 525 mPa leads to a decrease of the hardness to 7 GPa. With increasing bias voltage a preferred 200 grain orientation and denser crystalline morphology are indentified. Regarding coating thickness uniformity, the coating deposited with -150 V shows the lowest deposition rate difference between the surface parallel and perpendicular to cathode. In addition, optical emission spectroscopy (OES) was used to investigate the effect of bias voltage and deposition pressure on intensity of Cr (357 nm) and Cr+ (283 nm) close to substrate during the coating process. In the studied parameter range, the change of the bias voltage shows a light effect on the Cr and Cr+ signal. The increasing of bias voltage from 0 V up to -150 V leads to decrease especially of non-ionized Cr. This effect can also be observed with increasing deposition pressure.

Versatile and reliable techniques for evaluation of hard thin coatings are necessary for the development and tribological assessment of new coatings. Several different experimental techniques such as a nano-indentation, scratch and wear tests are normally used. However, most of the tests cannot assess properties of coating, substrate and interface independently.

We have proposed a new type of Micro Slurry-jet Erosion test (MSE), i.e. a solid particle impact erosion test for swift evaluation of coatings. In this study, the potential of our developed MSE test is demonstrated for the evaluation of different kind of vapor deposited coatings including PVD or CVD-TiN, TiC, TiAlN, CrN and DLC. Slurry containing 1.2 μm alumina particles was mixed with compressed air in the nozzle and eventually impacted on test materials at high velocity up to 100 m/s. The cross-section of the nozzle exit was a square of 1 x 1 to 3 x 3 mm² . The impingement angle was 90 degrees. Wear progressions in depth profiles of the wear scar were measured. The wear depths vs. the amount of impacting particles or test time curves show the individual behavior depending on the properties of coatings, substrate and interface. Their wear rates show a huge difference in the various coatings. The new MSE test generates highly reproducible results and is very sensitive to the quality of the coatings.

So our conclusion is that MSE tests can evaluate (1) coatings and substrate strength, (2) strength or property distribution in the direction of depth from the surface, (3) coating thickness based on strength or functional aspect, (4) interface layer or zone strength, (5) change of substrate properties according to deposition process. Consequently, the MSE test is highly suitable as a screening test when evaluating single and multi-layered coatings, thin coatings with gradients, coating/substrate interfaces, etc.

TiAlN and CrN are using widely in many industrial applications. In this work, TiAlN/CrN multilayer films deposited onto M2 high-speed steel and were examined by closed field-unbalanced magnetron sputtering (CFUBMS). Process parameters of coatings were determined according to Taguchi L₄ (3²) method. The structural properties of coatings have been analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectrometry (EDS). Besides, the mechanical and tribological properties were determined by using microhardness tester and pin-on-disc test, respectively. Experimental results showed that TiAlN/CrN multilayer films improved noticeably tribological properties of cutting tools.

ZrO₂(N) thin films were deposited on AISI 304 stainless steel substrate using hallow cathode discharge ion-plating (HCD-IP). Our previous study [1] indicated that the adhesion of ZrO₂ thin film on stainless steel was poor due to low wettability. The objectives of the present study were to understand the phase transition of ZrO₂(N) thin films with increasing nitrogen contents and to provide a feasible approach to manufacture ZrO₂(N) coatings on stainless steel with excellent corrosion resistance and good adhesion. By maintaining oxygen flow rate at 10 sccm and adjusting nitrogen flow rate, ranging from 0 to 12 sccm, the compositions and phase ratios of the ZrO₂(N) thin films can be controlled. With increasing nitrogen flow rate, the XRD patterns showed that the phase content of c-ZrO₂ increased while that of m-ZrO₂ decreased, and then ZrN phase increased. The N solubility limit in ZrO₂ for the formation of ZrN was 8.8 at% for the as-deposited thin film. After annealing in vacuum, different phase transitions were found for the specimens. At higher nitrogen content, phase separation of ZrN from c-ZrO₂ occurred in the specimens. The corrosion resistance of the ZrO₂(N)-coated stainless steel specimens was evaluated by potentiodynamic scan in both 5% NaCl and in 1N H₂SO₄ solutions, and salt spray test was employed to assess the durability of the films. Corrosion resistance was associated with film packing density and major phases. The results showed that the HCD-IP method can effectively overcome the surface wetting problem of ZrO₂ on stainless steel, and hence the ZrO₂(N) coating on stainless steel possesses excellent adhesion and corrosion resistance. ZrO₂(N) thin films containing ZrN ranging from 14.4 % to 28.8 % were found to have better corrosion resistance than pure ZrO₂ thin films.

[1] Jia-Hong Huang, Tzu-Chun Lin, Ge-Ping Yu, Surf. Coat. Technol.,206(2011)107.

Plasma Enhanced Chemically Vapor Deposited a-SiC:H thin films are compelling materials for both semiconductor nano-electronic and MEMS/NEMS technologies due to the extreme chemical inertness of this material and the ability to tune a variety of material properties across an extreme range of values. As one example of the latter, we demonstrate that the hardness of a-SiC:H thin films can be varied from 0.5 to 40 GPa. Utilizing Fourier Infrared-Transform Spectroscopy, we additionally show that this remarkable range in materials properties is achieved primarily via the incorporation of terminal hydrogen groups which lowers the overal connectivity of the Si-C network bonding. We find that once the average network coordination number for Si and C falls below 2.6, the Si-C network becomes under constrained and there is a loss of rigidity percolating through the system. This rigidity limit constrains the range of materials properties that can be achieved in the Si-C system.

In comparison to pure hydrogenated amorphous carbon coatings (a-C:H), the metal-modified variants (a-C:H:Me) are known to have better adhesion to the substrate due to lower residual stresses and more stable friction in dry sliding against steel. For this reason, particularly in higher loaded applications, a-C:H:Me is usually preferred over a‑C:H, even though hardness and hence abrasion protection tends to be lower. Dependent on the deposition parameters, the properties of a‑C:H:Me can be varied over wide ranges: From metal-like and carbide-like to amorphous-carbon-like. On the one hand this allows to a certain extent the adaption of the coating’s mechanical properties to the load-carrying and overload capability required by the targeted application. On the other hand, this offers the opportunity to use these coatings for the realization of tailored friction properties.

In the present study, tungsten-modified hydrogenated amorphous carbon coatings (a‑C:H:W) were deposited on hardened and tempered tool steel using an industrial scale coating machine. The applied coating technique was reactive magnetron sputtering of a tungsten carbide (WC) target in argon-acetylene atmosphere. To improve the adhesion to the substrate, a thin chromium layer and a WC intermediate layer were deposited by arc evaporation and magnetron sputtering, respectively. For the deposition of the a‑C:H:W functional layer, four process parameters, namely cathode power, bias voltage, process gas pressure and argon-to-acetylene flow ratio were varied according to a 2⁴ factorial design.