ICMCTF2013 Session B4-2: Properties and Characterization of Hard Coatings and Surfaces

In this presentation we discuss the latest research conducted at Southwest Research Institute^® (SwRI^®) in depositing diamond-like carbon (DLC) and other functionalized coatings on the inner surface of long pipes/tubes using plasma immersion ion deposition (PIID). Deposition of hydrogenated DLC of a few micrometers on the external surface of three-dimensional parts has been conducted for many years, and the PIID technology, a plasma-enhanced chemical vapor deposition (PECVD) process, has found some limited success for industrial applications. However, it is challenging to deposit these coatings on the inner surface of long pipes/tubes due to the difficulty of plasma generation at high aspect ratios. SwRI has developed technologies and demonstrated that tubes/pipes up to 25m long (100 mm in dia.) and as small as 16mm in dia. (by 3m long) can be coated with DLC. In addition to the “standard” DLC coatings for increased abrasion/erosion resistance, corrosion resistance and low friction, other surface functional coatings have been developed to obtain other physical properties including hydrophobicity, anti-scaling, and anti-icing. In this presentation, we will discuss the method for generating plasma inside the pipes, the characteristics of the plasma, and the microstructural, mechanical, and physical properties of various deposited films. The practical application examples will also be presented.

In this work, approximately 20 μm thick TiN-TiAlN erosion resistant coatings were deposited on turbine blades using industrial arc ion plating (AIP), and their surface morphology, microstructure, and mechanical properties were investigated. The film microstructure consists of the laminated TiN and TiAlN with varied architectures. The X-ray diffraction confirmed the mixture of cubic TiN and TiAlN phases with the lattice parameters of 0.424 nm and 0.415 nm, respectively. The film hardness was measured to be 2600 HV by nanoindentatation.

Nanocomposite coatings composed of two phases with atomically sharp phase boundaries show interesting mechanical properties. These properties often originate from their typically high interface to volume ratio.

The topic of this talk addresses interfacial properties of two-dimensional bilayer systems, which are used as model systems to describe the interfaces occurring in nanocomposite coatings. The presented systems are TiN interfaces in contact with silicon (Si), silicon nitride (Si₃N₄) and aluminum nitride (AlN). The primary tool used to analyze the interfaces of bilayer systems is X-ray Photoelectron Spectroscopy (XPS) with emphasis put on the shake-up feature of the Ti 2p photoelectron line. Shake-ups in TiN are observed as an additional peak on the lower binding energy side of the energy lines of the Ti 2p orbitals.

Angle-resolved XPS (AR-XPS) and X-ray Photoelectron Diffraction (XPD) results were used to interpret the crystalline structure of the different TiN/AlN and TiN/Si₃N₄ bilayer systems. The revealed interface properties show a correlation between the shake-up intensity and the interface morphology, oxygen content, interfacial charging and the shake-up energy. The results indicate that AlN grows crystalline on single-crystalline TiN, while Si₃N₄ only shows a crystalline growth behavior in the first 0.6 nm. The crystalline growth of Si₃N₄ in the initial stages is hindered in cases, where a bias voltage is applied to the substrate during Si₃N₄ deposition.

It is shown that the increase in the shake-up intensity is correlated to intrinsic and extrinsic interface charging. The obtained results, in combination with theoretical structure models from literature, show that in one to two monolayer thick interlayers a build-up of intrinsic interface charging is unlikely.

The inclined impact test was employed for assessing diamond coatings (DC) adhesion. This test has been established as a very efficient method for characterizing quantitatively the film adhesion, since the oblique loading direction induces shear stresses into the film and its interface to the substrate. Inclined impact tests were conducted on diamond coated specimens at various loads and cycles. The related imprints were evaluated by confocal measurements and EDX micro-analyses. According to the attained results, after a certain number of impacts dependent on the applied load, damages in the film interface occur resulting in coating-substrate detachment. In this way, coating residual stresses are released leading to a film swelling (bulge formation). In the bulge, the film residual stresses are eliminated. Furthermore, the bulges are destroyed after a restricted number of repetitive impacts. The development of swellings on diamond coatings due to film detachments during the inclined impact test can be effectively described by appropriate FEM simulations.

New generation of Ti-free coatings like as CrN, CrAlN, CrAlCN and CrAlYN coatings have combination of high temperature abrasion and oxidation resistance with low thermal conductivity. So these kinds of coatings have applications in industries like as automotive and airspace for protection of special alloys and in high speed dry machining tool coatings. But for using these coatings to their full potential beside metallurgic methods, coatings’ structural engineering is essential. Besides coatings with multilayer nanoscale structures compared to simple ones have more hardness and better oxidation resistance. In this study, two CrY, one Cr and one Al targets used to deposit CrAlYN/CrY thin film on M2 steel in nine runs with different configurations by unbalanced closed field magnetron sputtering system (CFUMBS). The variables were targets voltages and pulse parameters were applied to the substrate and reactive gas flow rate was constant. Coatings’ composition, morphology and structure were analyzed by a variety of techniques including SEM, and XRD.

ZrO₂ is an excellent corrosion protective coating for metal substrates especially in salt water. In our previous study [1], a wettability problem was encountered when depositing ZrO₂(N) thin films on stainless steel substrate. Instead of directly depositing ZrO₂, the oxide coating can be produced by oxidizing ZrN thin film which has no wetting issue on stainless steel. The purpose of this study was to produce ZrO₂ by heat treating ZrN thin films on stainless steel substrate and investigate the oxidation mechanism of ZrN films in vacuum. ZrN thin films were deposited on Si and 304 stainless steel substrates using hollow cathode discharge ion-plating (HCD-IP), and were annealed in vacuum (5 x 10^-6 Torr) at temperatures ranging from 700 to 1000°C and over durations ranging from 1 to 4 hr. The result showed that ZrO₂ was grown on top of ZrN that remained as the major phase (~ 60%) in the films even annealing at 1000°C for 4 hr in vacuum. This was attributed to the fact that the surface oxide was protective in vacuum, and therefore the diffusion of oxygen was hindered and remained an intact surface. Retained ZrN could be observed on the surface layer in stainless steel-based specimens even annealed at 800°C for 4 hr. After 500-hr salt spray test, the ratios of corrosion area for all the oxidized ZrN films were less than 0.2%, indicating an excellent corrosion resistance. Thus, by heat treating ZrN thin film in vacuum, ZrO₂ can be grown from ZrN without peeling and crack formation, which may provide good corrosion protection. Moreover, the stress in ZrN thin films could be relieved without significant change in properties as the specimens were annealed at 1000°C for 1 hr in vacuum.

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

We investigated the structural and mechanical properties of ternary alloys thin films Zr_1-xTa_xN with 0 ≤ x ≤1 deposited at Ts=300°C by reactive dc magnetron co-sputter deposition from individual Zr and Ta targets in Ar+N₂ plasma discharge. The working pressure was varied in the 0.19-1.0 Pa range by increasing the Ar flow from 12 to 68 sccm, while maintening the N₂ partial pressure before the onset of the compound target mode. The structural properties of the ternary Zr_1-xTa_xN compounds were characterized by X-ray Diffraction (XRD) and X-ray reflectivity (XRR), whereas the picosecond ultrasonics and Brillouin light scattering techniques were employed to measure their acoustic and elastic properties as function of the chemical composition and the working gas pressure. The thermal stability of these Zr_1-xTa_xN films is also studied, based on their structural and mechanical response upon vacuum annealing at 850°C for 3h.

In this study, we analyzed the high temperature tribological behavior of CrAlTiN coatings deposited on WC substrates by low cathodic arc technique. The coatings chemical composition, Al 31 at.%, Cr 16 at.%, Ti 7 at.% and N 46 at.%, and the bonding state were evaluated by X-ray photoelectron spectroscopy. The mechanical properties of the coatings were studied by scratch-test and nanohardness depth sensing indentation (hardness approx. 38 GPa). The morphology of the coatings surface, ball scars, wear tracks and wear debris as well as the oxidized samples was examined by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The structure and oxidation resistance were analyzed using high temperature X-ray diffraction (XRD) and thermal gravimetric analysis (TGA), respectively, up to 1300 °C. Wear testing was carried out using a high temperature tribometer (pin-on-disc) with alumina balls as counterparts. The evaluation of the friction coefficient with the number of cycles (sliding distance) was assessed at different temperatures and the wear rates of the coatings and balls were determined; the maximum testing temperature was 800 °C. The coating showed an excellent thermal stability and wear resistance. The friction reached a maximum at 500 °C and then decreased, whereas the wear rate was negligible up to 600 °C and increased significantly at higher temperatures.

To analyze the worn surfaces, several surface analytical techniques were applied: Raman spectroscopy was used to analyze the wear debris and wear track surface, chemical profile of the top surface part of the wear track was obtained by X-ray photoelectron spectroscopy (XPS), and the wear track cross-sections prepared by focused ion beam (FIB) were directly observed by transmission electron microscopy (TEM). Two findings emerge from our study: i) the surface of the wear track is not oxidized even after the sliding test at 800 °C, and ii) the coating nanolayered structure produced by sample rotation acts as an effective barrier to crack propagation at the nanoscale.

Among other material properties like elastic modulus E, hardness H is widely used in literature to mechanically characterize thin films or coatings with respect to their resistance to plastic deformation. However, hardness is not a generic material parameter as it can be biased by surface structures (e.g. surface roughness) or other material properties (e.g. intrinsic stresses). For instance, it will be shown how simple surface roughness can lead to false apparent ultra-hardness results. Therefore, this work will show that its benefit for the mechanical characterization of coated or treated surfaces is limited. In addition, hardness is not a physical parameter and, thus, cannot be used for both engineering and optimization of coatings by means of model-driven simulations. Possible alternatives like yield strength will be highlighted instead and their advantages over hardness will be explained.

Transition metal carbides exhibit superior mechanical and tribological properties at a wide range of environmental conditions and contact pressures. More recently, vanadium carbide coatings have been considered for a number of industrial applications (e.g. automotive components, cutting tools, ball bearings) due to their high corrosion resistance and mechanical stability at elevated temperatures. While some studies have provided significant new insights on deposition methods of vanadium carbides, the friction and wear mechanisms of these coatings have received little attention. The goal of this study is to provide an excessive understanding of the mechanical and microtribological properties of various vanadium carbide-based (VC_1+x) coatings. More generally, we are studying the influence of V:C ratio over a wide range contact pressures. The coatings are prepared using non-reactive d.c. magnetron sputtering with a segmented VC/graphite target (i.e. target diameter of 75 mm, 500 W target power, substrate temperature < 150°C, and Ar gas pressure of 0.6 Pa). The resulting V:C ratios vary between 1:1 and 1:3. The microstructures of the as deposited coatings are characterized using X-ray diffraction and cross-sectional focused ion beam imaging, while elemental analysis is performed by means of X-ray photoelectron spectroscopy, electron probe microanalysis, and micro-Raman spectroscopy. Mechanical properties measurements show that the hardness, H, of the coatings decreases with increasing the carbon concentration (i.e. H ranges between 1500 and 3100 HV for the low and high vanadium concentration respectively), which correlates well with the adhesion results obtained from scratch tests. However, reciprocating microtribological tests, performed with varying normal loads between 120 mN and 1 N, reveal higher friction values and increased wear with the high vanadium content coatings. This sliding behavior is attributed to differences in the third body formation and velocity accommodation modes, which are analyzed ex situ by means of micro-Raman spectroscopy and atomic force microscopy.

Keywords: vanadium carbide, non-reactive d.c. magnetron sputtering, microtribology, third bodies, velocity accommodation modes