K. Mohammed Jasim

Metal-ceramic functionally gradient material produced by laser processing

A technique of injection with a powder feed of a mixture of metal + ceramic which combines the processes of laser alloying, cladding and injection, has been applied to study the feasibility of using a continuous wave CO2 laser to produce a functionally gradient material. A 2 kW CO2 laser has been used to produce, on a nickel alloy substrate, single alloy/clad tracks and three totally overlapping clad tracks using powder mixtures of Al-10 wt% SiC, Al-30 wt% SiC and Al-50 wt% SiC, respectively. The variation of composition and structure with position in the processed material has been investigated with reference to the effect of processing traverse speed and the powder feed rate.

PULSED LASER PROCESSING OF THERMAL BARRIER COATINGS

Results are reported of the effects of surface melting (sealing) produced by a 1 kW laser in pulsed mode on the structure of plasma-sprayed 8wt% yttria partially stabilized zirconia (YPSZ); pulse lengths in the range of 1 to 90msec were used. Smooth surfaces were produced with shallow cracks at values of laser energy 5 to 40 J. Comparison of the data is made with results obtained by sealing using continuous wave CO2 laser processing.

Characterization of plasma-sprayed layers of fully yttria-stabilized zirconia modified by laser sealing

Abstract The effect of laser surface melting of plasma-sprayed layers of 20wt.% yttria-stabilized zirconia was investigated. The plasma-sprayed material consisted of approximately 90 mol% cubic (c) phase and approximately 10 mol% tetragonal (t) phase; on laser sealing at relatively high specific energies the proportion of the c phase increased. No t′ phase was observed either in plasma-sprayed or sealed layers. The microstructure of the sealed layers was of cellular or dendritic morphology depending on the processing parameters. Cracks formed during sealing penetrated through the sealed thickness and, in some cases, even through the unsealed region (plasma-sprayed zone) down to the bond layer.

Pulsed laser sealing of plasma-sprayed layers of 8 wt % yttria stabilized zirconia

Results are reported of the laser surface sealing of plasma-sprayed layers of 8 wt% yttria partially stabilized zirconia (YPSZ) using pulsed treatments with powers of 0.4 and 1 kW. The structural features of the processed material were examined for a range of laser processing parameters including preheating, processing temperature and power density. By controlling the processing parameters it was possible to produce pulsed laser sealed layers of similar, or even better, quality than those produced by a continuous wave laser.

Operating regimes for laser surface engineering of ceramics

Results obtained with CO2 lasers on the effect of processing parameters were used to determine the operating regimes for laser sealing and laser cladding of ceramics with particular reference to zirconias. The specific energy required for laser cladding is higher by at least two orders of magnitude than that required for laser sealing of plasma-sprayed ceramics at a given power density.

SEM studies of wear debris in Al-Si alloys

In the present letter we re-examine the wear debris in hypoeutectic, eutectic and hypereutectic Al−Si alloys. Wear debris covered after the wear run provides a means of further development. Debris was observed to be generated in this set of experiments within the first few minutes of the test run

Effect of mechanical properties on the adhesive wear of metals

There have been consistent efforts to understand the wear behaviour of materials. Many of the investigations were carried out with the aim of correlating wear behaviour with the basic properties of the material in order to determine the property that most controls the wear process. Since wear is a complex phenomenon, not yet fully understood, and many types of wear exist, such correlations must be specific in giving the particular test conditions. Since two materials are involved in wear, and the effects are confined to regions very close to the interface, the correlations must in some way consider the relative surface properties. Many investigators [1-3] have been successful in relating wear resistance, especially to abrasive type of wear, to the hardness of the materials to show a nearly linear variation. Correlations with other properties have been limited in extent. Theories of wear were proposed a long time ago by Finkin [4], Merchant [5], Feng [6], Bowden and Tabor [7] and Kerridge [8], and have been modified to apply to many wear situations. Often results seem to indicate [9] that wear depends only on the conditions bearing load, sliding speed and relative hardness of the softer surface. Evidence has also been obtained [5, 7] to show that it is more the yield stress of the softer surface than any other property that controls wear. Burwell [2] proposed a model for calculating the wear rate of metals, in which the relative hardness was more important than any other property, although data obtained on the basis of this model were highly scattered. The model of Archard [9] was not applied to the case of pure metals. The most significant correlation seems to be that of Rabinowicz [10], in which wear was shown to be related to the surface energy. It is often believed that the friction at the wear interface is closely related to the wear behaviour. However , wear situations can exist in which the wear rate can vary abruptly with no significant change in the friction coefficient. However , a recent study has [11, 12] shown that the coefficient of friction measured in a pin-on-disc machine for a number of pure metals bears a predictable correlation with several mechanical properties of the metal. However , correlations do not seem to have been made for metals for their wear behaviour under purely adhesive conditions. It has been recognized [13] that nearly all materials tend to form a wear-resistant surface layer under adhesive conditions, to justify the comment that wear resistance is more a function of the properties of the surface layer than that of the bulk. It was therefore thought that correlating the properties of the metal with the adhesive wear behaviour would yield interesting results, and this was attempted. Part of the results obtained are summarized in this letter. Cylindrical wear pins 5 mm in diameter were obtained from rods of the desired metals (Table I). One flat end of the pin was ground with 600-grade emery and then slid on a rotating highly polished steel surface of a hardness 50 H R C machine for a total of 30 min. The bearing load used was 5 N and the relative sliding speed at the wear interface was 1.25 m s -1. Steady-state wear rates were calculated from the weight measurements and expressed as volume worn per unit sliding distance (cm 3 cm-1). The measured wear rates in all of the metals studied for the conditions of the experiment were-of the order of 10-9-10 .8 cm 3 cm -1 and are considered