Deepak G. Bhat

Properties and performance of commercial TiCN coatings. Part 2: tribological performance

Tribological testing has been carried out on a range of commercial PVD and CVD TiCN coatings deposited on WC/Co, which were previously characterised in detail. Single-pass scratch tests were used to characterise adhesion, and pin-on-disc testing was used to determine friction coefficients and wear rates, respectively. The critical load for coating detachment depends on coating thickness, as expected for PVD coatings, but is unexpectedly low for a medium-temperature CVD layer. The friction coefficient against WC balls varies considerably between coatings produced by different processes, as does the wear rate. Maintaining the carbon content of the exposed surface during wear is critical in the reducing friction coefficient and wear rate after the run-in process is complete.

Properties and performance of commercial TiCN coatings. Part 1: coating architecture and hardness modelling

Abstract Several PVD and CVD TiCN coatings on a cemented carbide substrate have been obtained from commercial suppliers and subjected to a range of analysis and characterisation techniques to determine the effects of coating composition and architecture on performance. The majority of suppliers produce coatings with a C/N ratio approximately equal to 1, although one supplier increases this to 2. For all the PVD coatings, the supplier initially deposits a layer of TiN to promote adhesion and then gradually increases the nitrogen content of the process gas to make TiCN with a graded composition running from pure TiN to TiCN over a few 100 nm of coating growth. This graded layer may or may not be capped with a layer of TiCN with fixed composition. The hardness behaviour of these layers is more complex than a simple, single layer on the substrate. In this study, an existing predictive hardness modelling analysis developed at Newcastle, UK has been extended to deal with coatings of graded composition. The potential for optimising the hardness of TiCN coatings by changing coating architecture is discussed.

Indentation hardness evaluation of cathodic arc deposited thin hard coatings

Abstract One trend in the development of wear-resistant vapour deposited coatings is to make them increasingly harder and thinner, by improvement and optimisation of the deposition processes. A complex interdependence exists between the individual properties of a coating and a substrate on the one hand, and those of the ‘composite’ coated system on the other. For example, system stiffness and hardness may vary with indentation depth according to different laws. There is a great need for quantitative modelling methods so that the design of coatings and multi-layered systems can be improved, and the choice of materials optimised. In the present study, various hard coatings produced by filtered cathodic arc deposition were characterised by micro-indentation and macro-indentation methods, and scanning electron microscopy. SEM was used to elucidate the fracture behaviour of these coatings, which exerts an important influence on their hardness performance. Hardness testing results were analysed using a newly proposed modification of the work-of-indentation model, an approach that was recently developed and applied to a range of coated systems. The new development of this model allows more accurate fitting of the empirical data, and yields an estimate for the ultimate coating hardness, and values of dimensionless materials parameters β 0 and X . These parameters describe the normalised depth and the degree of abruptness at which the hardness transition from coating to substrate occurs, and are related in a complex way to the ductility and toughness of the thin film and the interface, as well as the substrates yield strength and hardening behaviour. In this study we use the model to interpret micro- and macro-indentation data, and to discuss the results of extrapolating the depth–hardness curve to the important region where the indentation depth lies is between 5 and 10% of the coating thickness, and composite hardness approaches the ultimate film hardness.

Characterization of large area filtered arc deposition technology: part I — plasma processing parameters

Abstract The transmission of metal–gas plasma flow generated by large area, rectangular dual-filtered-arc source is investigated. The characteristic parameters of plasma flow such as ion current yield and inter-electrode voltage drop vs. gas pressure, are established for the deposition of TiN coatings. The uniformity and productivity of coating deposition on complex parts in a 3D operational volume is determined. It is found that ion current yield increases from 6 to 10 A with increasing argon pressure in the range of 2–7×10 −2 Pa. It results in a deposition rate on the order 1–2 μm/h in a double rotation mode. It is shown that a thickness uniformity of ±10% or better can be achieved in a programmable, vertical scanning mode.

Coatings for Cutting Tools

Surface technology for wear property enhancement is changing design concepts. The changes are quite pervasive. They encompass productivity limits, bulk substrate specification, as well as tribological considerations. This brief account discusses commercialization of modern techniques in one such area. The interesting part of this story is really not the technical development itself, but rather the impressive potential improvement in service life.

Characterization of large area filtered arc deposition technology: part II — coating properties and applications

Abstract A dual-filtered cathodic arc deposition process was used to synthesize a variety of hard coatings on polished substrates, using large-area filtered-arc deposition (LAFAD) technology. The surface morphology showed that the coatings were free of macro defects or inclusions, and there was no degradation of the initial surface finish. Mechanical properties of the coatings deposited were measured by a nanoindentation technique. A duplex (heat treatment+deposition) process was used to deposit a unique multilayer hard coating on H-13 steel core pins used in aluminum die-casting application. Extensive characterization of the coated pins showed that the coating improved the erosion/corrosion resistance, as well as thermal cracking resistance, of the steel by nearly one order of magnitude over commercially used PVD coatings.

Thermal cracking behavior of multi-layer LAFAD coatings on nitrided die steels in liquid aluminum processing

Thermal cracking is the primary mode of failure of large steel dies in aluminum die casting of automotive engine blocks or transmission housing. This cracking is due to the large thermal shock experienced by the die steel when it is quenched by cold water after a die casting cycle. The propagation of cracks depends on the severity of the thermal fatigue cycle (peak and range), that is caused by die surface being in contact alternatively with the liquid melt at 700°C and cold water at room temperature. Previous work at The Ohio State University has shown that single layer hard PVD and CVD coatings do not protect the die steel surface from cracking. On the other hand, they may enhance cracking. This paper presents an interesting multi-layer duplex coating approach with a nitriding H-13 steel substrate and a multi-layer, hard outer film applied by LAFAD technique that prevents reaction with the liquid melt, and alters the thermal fatigue behavior. Results of thermal cycling tests indicate that the multi-layer duplex coating system helps reduce the density of the thermal cracks.

Dissolution and soldering behavior of nitrided hot working steel with multilayer LAFAD PVD coatings

Soldering and corrosive dissolution are the primary causes of die non-performance in permanent mold and die casting production of net shape parts for the transportation industry. At worst, these corrosive reactions result in core pin or sprue dissolution, and at the least, some soldered cast metal remains behind when the casting is ejected. The objective of this work was to explore the possibility of surface engineering of the die steel surface to reduce the dissolution and soldering tendencies of the surface. This paper presents a duplex surface treatment approach for dissolution resistance that consists of nitrided hot working die steel substrate coated by multi-layer titanium-based coatings applied by the large area filtered arc deposition technique. The dissolution behavior of the coating candidates is evaluated by measuring weight loss after dipping them in molten aluminum for a predetermined time. Results are compared with those of single layer coatings and surface treatments. The chemisorption and adhesion behavior is evaluated by measuring ejection force of the soldered material. The duplex coating is found to significantly reduce both the dissolution and adhesion tendencies of the coated steel surface.

Effects of deposition parameters on the structure of AlN coatings grown by reactive magnetron sputtering

AlN films were deposited on microscopy glass slide and silicon (111 orientation) substrates by reactive ac magnetron sputtering using two nitrogen concentrations and three discharge powers of 1.5, 2.5, and 5.0kW. X-ray diffraction studies showed that films prepared on glass and Si substrates were of hexagonal wurtizite phase. Films on Si substrates also contained small amounts of the cubic phase of AlN besides the predominantly hexagonal wurtizite phase. AlN coatings on glass substrates were textured towards the (00∙2) plane; this preferred orientation of crystals was found to decrease with increase in sputtering power. Scanning electron microscopy studies showed that AlN films prepared at higher nitrogen concentration have a microstructure consisting of pebblelike crystals, some of which were hexagonal in shape. The crystal size in the coatings increased with sputtering power and was in the range of 70–230nm.

Study of Airborne Dust Emission and Process Performance During Dry Machining of Aluminum-Silicon Alloy with PCD and CVD Diamond-Coated Tools☆

The generation of a fine mist of cutting fluid during conventional wet machining and the associated environmental and operator health concerns make environmentally benign machining and manufacturing a major research thrust both in the scientific and manufacturing communities. In this context, high-speed machining in combination with environmentally benign methods makes diamond-coated tooling a unique candidate for dry machining. Diamond coating research in the past few decades has resulted in new products, one of them being diamond-coated carbide tooling. Diamond in polycrystalline diamond (PCD), chemical vapor deposited (CVD) thin-film (polished and unpolished) and thick-film forms offers unique advantages for dry machining. This paper presents the correlation between diamond tool morphology, machining parameters, nonferrous workpiece properties, and particulate emission in dry machining. These findings provide an important benchmark to gauge the true benefit of diamond tools for dry machining.