R. Nimmagadda

Structure and property relationships in microlaminate Ni-Cu and Fe-Cu condensates☆

Abstract Microlaminate metal matrix composites offer considerable potential as high strength high toughness materials with isotropic properties in the plane of the sheet. In this paper we deal with the preparation of Fe-Cu and Ni-Cu microlaminate composites by electron beam evaporation from two sources and alternating deposition onto a rotating substrate. The thickness of the laminae was varied by changing the evaporation rates of the metals and the speed of rotation of the substrate. The deposits were characterized by optical microscopy, scanning electron microscopy and electron microprobe analysis to study the thicknesses of the laminae and interdiffusion of elements between the layers. X-ray diffraction was used to study the composition of the laminates. The microhardness and tensile strength at room and high temperatures of the laminates were studied and correlated with the thicknesses of the laminae and the microstructure. The results showed very marked increases in strength and hardness as the size of the laminae decreased below about 2 μm. Superplastic behavior was observed at a certain thickness of the laminae, strain rate and temperature.

Friction and adhesive wear of titanium carbide and titanium nitride overlay coatings

Abstract The coefficient of friction and self-adhesive wear were studied for hard overlay coatings (4–8 microm thick) of TiC and TiN deposited on 304 stainless steel, titanium and aluminum discs and 440 C stainless steel riders by activated reactive evaporation process. The hard overlay coatings were characterized by X-ray diffraction and indentation hardness techniques. The frictional and adhesive wear behavior were evaluated using a standard pin-on-disc tribotester in dry and lubricated conditions. The coefficient of friction and the wear were very much lower when the test couple consisted of a hard coating rubbing against a hard coating than when one or both of the components were uncoated metals, even in dry (unlubricated) conditions. In some instances of the hard-hard couple, very low coefficients of friction (0.05–0.1) and very low wear, i.e. wear groove depths less than 1 × 10−5 in, were observed for test conditions of a run distance of 500 m under a load of 0.4 kgf in dry conditions. From examination of the wear scar the failure mode of the coating was observed to be microfragmentation resulting from the initiation and propagation of microcracks at the rider-disc interface.

Influence of ion implantation and overlay coatings on various physico-mechanical and wear properties of stainless steel, titanium and aluminium

Abstract This work represents the data on the wear behaviour of 304 stainless steel, commercial grade titanium and commercial grade aluminium without and with different surface treatments, namely ion implantation of boron and nitrogen and overlay coatings of titanium carbide and nitride. The surface treatments were characterized by phase identification, hardness, bend strength, as well as adhesion of overlay coatings. Wear properties were evaluated in adhesive, erosive and abrasive modes of wear. The experimental results showed that surface treatments produced measurable changes in hardness and strength. The results of adhesive wear tests indicated that the wear resistance of all the substrate materials can be considerably improved by overlay coating with superhard materials in dry as well as lubricated test conditions. Ion implantation resulted in improvement of wear properties for only a limited regime of adhesive wear under lubricated conditions and for the abrasive mode of wear. Overlay coatings produced a marked improvement in abrasive wear tests under lubricated conditions on all substrate materials.

Microstructures of TiN and Ti2N deposits prepared by activated reactive evaporation

Abstract Titanium nitride has an important application as a coating on cutting tools to extend their lifetime. In this study various phases in the Ti-N system were deposited by the activated reactive evaporation process. The influence of process variables—evaporation rate, N 2 partial pressure and deposition temperature—on the phases present, their morphology and hardness, was studied using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. As the ratio of the evaporation rate of titanium to the partial pressure of N 2 decreases, the deposit changes from titanium to Ti 2 N to TiN including two-phase mixtures. As the deposition temperature is increased the grain size increases markedly, from about 0.5 μm at 550 °C to 20 μm at 1000 °C, and the grain morphology changes from a faceted to a smooth topography. The hardness of the deposit is influenced primarily by the presence of the Ti 2 N phase which produces the highest hardness levels.

Transmission electron microscopy studies of TiC and VCTiC deposits prepared by activated reactive evaporation

Abstract The activated reactive evaporation technique is used for processing carbide alloy coatings on tools. Improved properties of these films can be developed if the relationships between processing parameters, microstructural features and mechanical properties are well understood. The main aim of the present investigation was to characterize the microstructures of TiC and (V,Ti)C films prepared by the activated reactive evaporation process at varying deposition temperatures (550–1000°C) and to relate these features to the hardness. The transmission electron microscopy technique was used for this purpose. Two characteristic and completely different grain morphologies are produced in these films, one at low temperatures (extremely fine grained with grain diameters of about 10 nm) and one at high temperatures (of more normal grain structure with micronsized grains). These are all equilibrium single-phase structures of TiC and (V,Ti)C respectively except for the low temperature deposit of (V-23 at.% Ti)C prepared at 550°C; (V-23 at.% Ti)C is a mixture of TiC, VC and free graphite phases. The hardness is superior in the large-grained structures.

Synthesis of titanium nitrides by activated reactive evaporation

Abstract In this paper we deal with the experimental results on the synthesis of titanium nitrides by activated reactive evaporation. In this process, titanium is evaporated by a transverse electron beam source in the presence of nitrogen gas. The vapor species were activated by a low voltage plasma. The experimental variables studied were the evaporation rate, the partial pressure of nitrogen gas and the substrate temperature. The deposits were characterized by microhardness, X-ray diffraction analysis, adhesion measurement, scanning electron microscopy and transmission electron microscopy. It was observed that the hard deposits were essentially a mixture of TiN and Ti 2 N phases. The deposition of hard coatings requires close control of the deposition parameters. A Knoop hardness of 2600–2800 HK (50 gf load) could be obtained under optimum conditions.

Structure and properties of refractory compounds deposited by electron beam evaporation

This work reports on the structure and properties of the refractory compounds TiC, ZrC, TiB2, ZrB2, TiCZrC, TiCTiB2 and TiCTiB2 Co. The deposits were prepared by direct evaporation of TiB2, ZrB2, ZrC, TiC and cobalt from single and multiple water-cooled copper crucibles using electron beam heating. TiC and ZrC deposits were also prepared by the activated reactive evaporation process. The vapors were condensed on a molybdenum or tantalum substrate at various deposition temperatures ranging from 650 to 1600 °C. The deposition rate was varied from 0.08 to 6 μm min−1 The deposits were characterized by optical microscopy, scanning and transmission electron microscopy, X-ray and electron diffraction and microhardness determinations. With direct evaporation the deposits contained decomposition products in addition to the parent phases. The composition of the deposits was dependent on temperature of deposition, composition of the evaporant billet and to a small extent the rate of deposition. Deposition temperature, rate of deposition and the composition of the deposit influenced the preferred orientation and the microhardness of the deposits. Surface and fracture cross section morphology and microstructure varied with deposition temperature. The data represent an extensive characterization of refractory compound deposits made by high rate physical vapor deposition processes.

High rate deposition of hafnium nitride by activated reactive evaporation (ARE)

Abstract Cemented carbide inserts are being successfully coated with HfN using the chemical vapor deposition process. Their machining performance is much better than that of uncoated tools. To coat high speed steels with HfN at low deposition temperatures and high rates, we have to resort to physical vapor deposition processes. This paper is concerned with the synthesis of HfN by the activated reactive evaporation (ARE) process and characterization of the deposits. Hafnium was evaporated from an electron beam source in the presence of the reactive gas, nitrogen, the vapor species being activated in the ARE process. The evaporation rate of the metal, reactive gas pressure, deposition temperature and the substrate material were the experimental variables. On stainless steel and high speed steel substrates HfN could be deposited, whereas on tantalum substrates, only mixtures of hafnium, Hf 3 N 2 , Hf 4 N 3 and HfN could be deposited. The microhardness of the deposits varied from 1850 to 2420 KHN.

Structure and properties of refractory compounds deposited by direct evaporation

Abstract This paper reports on the structure and properties of refractory compound deposits of TiB 2 , ZrB 2 , ZrC, 55% TiB 2 −45% ZrB 2 and 60% ZrC−40% ZrB 2 . The deposits were prepared by direct evaporation of TiB 2 , ZrB 2 and ZrC from water- cooled copper crucibles using an electron beam as the heat source. The vapors were condensed on a Mo substrate which had a temperature gradient ranging from 650°C to 1600°C. The deposition rate was 1–3 μm min -1 , and the thickness ranged from 25 to 75 μm. The deposits were characterized by optical and scanning electron microscopy, X-ray diffraction analysis, energy-dispersive analysis and microhardness determinations.

Machining studies on coated high speed steel tools

Abstract This paper reports studies on the machining of type 4340 steel heat treated to Rc 33 by high speed steel tools (type M 42) coated with TiC using the activated reactive evaporation process. The coated tools exhibited a tool life of 300–800% higher than the uncoated tools under continuous cutting conditions. The principal failure mode was flank wear. The cutting forces were measured and were found to be 50% lower for coated tools than for uncoated tools.