Dennis T. Quinto

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.

Pulsed plasma-assisted PVD sputter-deposited alumina thin films

Abstract Crystalline γ-Al 2 O 3 thin films have been deposited using reactive sputtering of aluminum targets. To prevent arcing, a dual magnetron configuration was used, while the targets were powered by a bipolar pulse power generator at 50 kHz. Scanning electron microscopy (SEM) revealed that films grown at 550 °C have a fine-grained morphology. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis revealed that approximately 80% of the film has been condensed and grown in the γ-phase crystal structure. Film hardness and elastic modulus values obtained by nanoindentation on ∼2-μm-thick films were measured as approximately 25 GPa and 350 Gla respectively at 300 mN load. Constant particle bombardment of the growing films was important and the film morphology could be changed from a strong columnar to a dense compact structure.

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.

Technology perspective on CVD and PVD coated metal-cutting tools

Abstract This review gives an account of the technological developments in hard coatings that have been commercially implemented in modern metal-cutting tools. Both CVD and PVD coatings have been successfully applied to cemented carbide metal-cutting inserts, in spite of the apparent competition between the two technologies. CVD coatings have evolved over the past 20 years with multilayer configurations that are still predominant today. PVD single-layer coatings have found niche applications, where conventional CVD coatings have not been particularly successful, and are expected to challenge standard CVD coatings with further advancements. Recently, a CVD/PVD layer combination has been commercially introduced. Scenarios for the development of newer coatings such as diamond and CBN are discussed. A failure mode diagram is proposed as a conceptual framework for metal-cutting tool design which takes into account the total system: the wear environment imposed by workpiece and machining parameters, corresponding tool failure modes and the synergy between hard coating and substrate.

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.

Comparison of the steel-milling performance of carbide inserts with MTCVD and PVD TiCN coatings

This study is a comparison of the performance of PVD-TiN-, PVD-TiCN-, and MTCVD-TiCN-coated milling inserts. It is shown that metal-cutting conditions and insert geometry dictate the optimum performance for each type of coating. The importance of compressive residual stress in PVD coatings in delaying the initiation of damaging cracks at the edge is evident in these results. Conditions in which thermal shock is not severe, such as dry milling or lower-speed wet milling and the use of sharp cutting edges, are shown to maximize the performance of the PVD-TiCN-coated tool. On the other hand, the MTCVD coating necessitates a strong cutting edge geometry on a tough substrate to obtain good performance, in wet milling. Practical productivity implications, including environmental concerns, are considered.

Mechanism of hot-shortness in leaded and tellurized free-machining steels

This investigation was aimed at understanding the mechanism of hot-shortness in AISI 12L14 + Te steels, which might lead to a hot-rolling practice to minimize the high yield losses in this grade. High temperature tensile tests showed a pronounced loss in ductility between 810 and 1150 °C, the embrittlement being most severe at about 980 °C. Electron microprobe studies confirmed thermodynamic stability data which indicated that tellurium occurs primarily as PbTe in this steel composition. SEM fractography revealed increasingly brittle, partially intergranular tensile fracture with a loss of ductility. Auger Electron Spectroscopy of samples quenched from the embrittlement temperature range indicated the formation of a thin film of PbTe on the grain boundary surfaces. All these results are consistent with a mechanism of liquid metal embrittlement by PbTe which has a melting point of 923 °C. Some theoretical considerations of this mechanism are discussed. The characteristic return to ductility above the embrittlement range suggests that rolling at temperatures above 1150 °C might minimize the hot-shortness problem. Results of limited hot-rolling experiments to study the incidence of surface cracking as a function of temperature support the above suggestion.