Prem C. Jindal

Adhesion measurements of chemically vapor deposited and physically vapor deposited hard coatings on WCCo substrates

Abstract Comparative measurements of adhesion of single-layer and multilayer hard coatings on cemented carbide substrates are made using scratch and indentation test methods. It is shown that the critical load L c in scratch adhesion is influenced by the hardness of the substrate and extraneously by the surface condition of the indenter and its coefficient of friction relative to the coating material. The average stress calculated from the width of the scratch channel is found to be a more meaningful adhesion parameter than L c . In the indentation technique, the approximate load for lateral crack initiation and the slope obtained from the indentation load-lateral crack length function are two useful adhesion parameters. The latter is demonstrated to be more discriminating in several test cases. It is also found that increased residual stress in physically vapor deposited TiN coating reduces both measured scratch and indentation adhesion parameters.

High temperature microhardness of hard coatings produced by physical and chemical vapor deposition

Abstract The microhardness of hard coatings of TiN and HfN prepared by chemical vapor deposition (CVD) and TiN, HfN, ZrN and TiAlN prepared by physical vapor deposition (PVD) were measured between room temperature and 1000°C. The microhardness of the PVD coatings was significantly higher than that of the CVD counterparts at room temperature but all microhardness values tended to converge at 1000°C. Higher N: Ti ratios were found in PVD TiN relative to CVD TiN. X-ray diffraction measurements indicated that the PVD coatings contained high residual compressive growth stresses associated with lattice distortion and very fine grain size, which were confirmed using transmission electron microscopy. Low residual stresses in high temperature CVD coatings are caused by thermal expansion mismatch between coating and substrate. Differences in grain morphology and crystal texture are attributed to varying conditions of energetic bombardment and deposition temperature in the several PVD methods employed. The faster decrease in microhardness with temperature in PVD coatings is caused by the high residual energy and finer grain size.

A new class of high performance PVD coatings for carbide cutting tools

Abstract A number of advanced PVD coating designs based on Ti–Al–N–C–B were evaluated in metalcutting. Monolayer PVD TiN, TiAlN, TiB 2 and different variants of TiAlN multilayer coatings were deposited on WC-6 wt.% Co hardmetal inserts. The coatings were applied either by cathodic arc processes or a high-ionization magnetron sputtering process. The coated tools were evaluated in milling of ductile and gray cast irons with and without coolant, and in turning of Inconel 718 and a hypereutectic AlSi alloy. The TiAlN-multilayer coated tools showed the best performance in dry milling applications; the TiAlN-monolayer coated tools performed better under wet milling. The observed results are consistent with a model that takes into consideration the inherent residual stresses within the coating, the stresses during machining, and the bonding strength of the coating layers to the substrate. In Inconel 718 turning, the TiAlN-multilayer coating showed some performance advantage over the TiAlN-monolayer and the TiN/TiCN/TiAlN-multilayer coating particularly at higher speed. In the turning of the aluminum alloy, PVD TiB 2 had performance advantage over PVD TiAlN and PVD TiN, which could be correlated with their relative hardness values.

Advanced PVD-TiAlN coatings on carbide and cermet cutting tools

Abstract Significant advances have been made in the process design and development of PVD-TiAlN coatings for carbide and cermet cutting tools. Higher plasma ionization in the TiAlN deposition process creates coatings with dense microstructure and excellent adhesion characteristics. The resulting new generation of PVD-TiAlN coatings provide increased productivity in a wide range of machining operations and work-piece materials. This paper discusses the characteristics of high-ionization TiAlN coatings and their performance in metal-cutting applications.

Mechanical properties, structure and performance of chemically vapor-deposited and physically vapor-deposited coated carbide tools

Abstract The relative merits of TiN coatings produced by physical vapor deposition (PVD) and chemical vapor deposition (CVD) on cemented carbide tools are examined by a comparison of microstructure, mechanical properties and metal-cutting performance of two hard metal alloys in the uncoated condition and after coating with either type. Intrinsic microstructural differences between PVD and CVD coatings are attributed to the differences in deposition process conditions. It was found that the transverse rupture strength (TRS) of the ground substrates decreased after CVD coating or high temperature treatment but remained relatively unchanged after PVD coating. The TRS values are correlated with the residual stress states of the TiN coating and the WC phase of the underlying substrate. The optimum condition for high TRS and minimum edge chipping sensitivity was obtained in the ground and PVD coated hard metal where the TiN coating and the WC phase were under relatively high residual compression. The best metal-cutting performance in both turning and milling tests was obtained with the substrate exhibiting high deformation resistance and the PVD TiN coating which had a higher microhardness and beneficial compressive residual stress.

Load dependence of microhardness of hard coatings

Abstract Vickers and Knoop microhardness measurements on hard coatings on WC-Co substrates were studied as a function of indentation load. It is shown that the increase in conventional Vickers hardness with decreasing load, i.e. the indentation size effect, can be derived by considering the elastic recovery in indent length on unloading. It further follows that the large increase in microhardness at light loads (up to 100 gf) depends inversely on the elastic modulus of the hard coating. Microhardness numbers based on Vickers or Knoop diagonal measurements are shown to have varying sensitivity to the elastic recovery effect. Depending on coating thickness, the microhardness- load profiles can indicate the existence of gradient residual stresses in the coatings as well as effects due to the coating-substrate interface and the substrate hardness.