J.-P. Hirvonen

Characterization of the surface oxide formed by excimer laser surface processing of AISI 304 stainless steel

Abstract We have used excimer laser processing in air to grow thick oxide layers on AISI 304 stainless steel and have characterized the resulting oxide layer using a variety of techniques including Mossbauer, Auger, and Rutherford backscattering spectroscopy, X-ray and electron diffraction, transmission electron microscopy, and nuclear reaction analysis. The resulting oxide, which is grown under rapid quenching conditions, has a spinel structure and a complex composition. In particular, a Cr to Fe ratio of approximately one was observed to persist throughout the oxide. Carbon incorporation at approximately 13 at%, presumably from surface contamination, and N at approximately 5 at% were also found.

Excimer Laser Surface Modification: Process and Properties

Surface modification can improve materials for structural, tribological, and corrosion applications. Excimer laser light has been shown to provide a rapid means of modifying surfaces through heat treating, surface zone refining, and mixing. Laser pulses at modest power levels can easily melt the surfaces of many materials. Mixing within the molten layer or with the gas ambient may occur, if thermodynamically allowed, followed by rapid solidification. The high temperatures allow the system to overcome kinetic barriers found in some ion mixing experiments. Alternatively, surface zone refinement may result from repeated melting-solidification cycles. Ultraviolet laser light couples energy efficiently to the surface of metallic and ceramic materials. The nature of the modification that follows depends on the properties of the surface and substrate materials. Alloying from both gas and pre-deposited layer sources has been observed in metals, semiconductors, and ceramics. Surface enrichment of Cr by zone refinement of stainless steel has also been seen. Rapid solidification after melting often results in the formation of non-equilibrium phases, including amorphous materials. Improved surface properties, including tribology and corrosion resistance, are observed in these materials.

Microstructure and mechanical properties of ion‐beam‐produced Fe‐Ti‐(N), Fe‐Ti‐(C), and Fe‐Ti‐(C,N) surface films

Ion‐mixed films of Fe53 Ti47 were produced by ion irradiating a Fe‐Ti multilayer structure on AISI 304 stainless steel. The ion‐mixed films were subsequently implanted with nitrogen, carbon, or both carbon and nitrogen. The microstructure following nitrogen implantation consisted of a bcc solid solution of iron and titanium and finely dispersed TiN precipitates. In the cases of carbon or carbon and nitrogen implantation, a two‐phase structure consisting of an amorphous matrix with TiC or Ti(C,N) precipitates was found. All these films initially possessed improved tribological properties as revealed by lowered friction and increased wear resistance. However, after an extended test of 1000 wear cycles, a reduced friction was only observed for the carbon or carbon and nitrogen implanted samples. The wear track on the dual implanted surface was extremely smooth, while the surface of the nitrogen‐implanted sample was partly worn through, causing the friction to increase to the level of the untreated sample. The improved tribological properties of the implanted films are attributed to an increase in surfacehardness. However, the surfacehardness is unable to explain differences between different implantations. In the case of the dual carbon and nitrogen implantation, improvements appear to be in part the result from an increased capability to accommodate plastic deformation. These conclusions are supported by transmission electron microscope studies of the wear tracks as well as by nanoindentation measurements.

Post deposition excimer laser processing of MoS x thin films

We have examined the effect of excimer laser surface processing and doping with Au on the mechanical, tribological, and bonding properties of MoS{sub {ital x}} thin films. We find that the effects of processing are manifested primarily in the surface of the films, but that there is also some film-substrate interaction during high fluence processing. The changes are sufficient to dramatically alter the wear life of the films. At low loads, laser processing alone reduces the run-in period and increases wear life. At higher loads, processing reduces wear life, although the run-in period is still short. These results are understood in terms of changes in the hardness of the surface of the films. Laser processing renders deposited films insensitive to high humidity storage. After laser processing, Raman spectroscopy shows changes in bonding to that characteristic of single crystal MoS{sub 2}. Laser doping with Au results in wear life comparable to or greater than that of unprocessed films even at the highest loads used. Thus, laser doped MoS{sub {ital x}} films show lower initial friction, comparable wear life, and may exhibit greater environmental stability than as-deposited films.

Tribology and mechanical properties of excimer laser mixed Ti--Si--C surface alloys

We have examined the wear and friction and surface hardness of the mixed phase Ti--Si--C alloy formed by excimer laser surface processing of Ti layers on SiC substrates. The friction between a ruby pin and the mixed surface shows a complex behavior depending on relative humidity, a behavior clearly moderated by the chemistry of the interface between the sliding pin and the surface. The friction is sometimes much lower and sometimes comparable to that between the ruby pin and the unalloyed substrate. Wear in the unalloyed case is characterized by fatigue fracture and flaking of the SiC surface which leads to abrasive wear of the ruby pin. In the alloyed case, a transfer film forms and even in the worst case, a smooth wear track results in the alloy and the pin is undamaged. The surface hardness is intermediate between that of the SiC and the unalloyed Ti surface layer. The wear results are understood in terms of changes in the grain boundary structure of the surface induced by the laser alloying process.

Laser Mixing of Titanium on Silicon Carbide

We have used excimer laser surface processing to melt and mix single Ti layers into the surface of polycrystalline SiC substrates. The mixing of Ti into the surface is very rapid and efficient. Examination of Rutherford backscattering (RBS) data for different mixing conditions shows the formation of a preferred composition at the Ti-substrate interface which propagates from the interface with further mixing. Reconstruction of the RBS spectrum indicates that the composition of the layer is Ti{sub 45}C{sub 37}Si{sub 18}. X-ray diffraction demonstrates the formation of Ti silicides and carbides in the surface region. Profiling of C in both mixed and uncoated samples by 5 MeV He{sup +} scattering demonstrates that laser processing of the SiC does not cause major changes in the stoichiometry of the substrate material. 6 refs., 3 figs., 1 tab.

Microstructure and tribology of ion-mixed Fe/Ti/C multilayers on AISI 304 stainless steel

A multilayered Fe/Ti/C structure consisting of eleven alternating sublayers, four Fe, four Ti and three C, was ion mixed on an AISI 304 stainless steel substrate with 400 keV Xe2+. Complete mixing was observed after an irradiation of 1 × 1017Xe/cm2 at 550°C. Electron diffraction revealed the formation of the compounds TiC and Fe3C and a small amount of an amorphous phase. Some samples were given a second irradiation with 5 × 1015Xe/cm2 at 0°C. The phases present following the second irradiation were TiC, α-Fe and an amorphous phase. Tribological and nanoindentation measurements revealed that both types of samples possessed similar hardness and friction properties. The ion mixed samples possessed an increased hardness and a decreased friction coefficient relative to untreated polished stainless-steel substrate. However, the wear life of the hot ion mixed sample was consistently longer than the wear life of the sample processed at both 550 and 0°C. These results are explained by differences in adhesive wear which result from differences in the chemical reactivity of the phases formed during ion beam processing.

MODIFICATION OF SURFACE MECHANICAL PROPERTIES OF HIGH CHROMIUM TOOL STEEL BY CARBON-IMPLANTED CODEPOSITED Fe—Ti FILMS

Abstract An iron-titanium film 300 nm thick was deposited on a tool steel (1.55% C, 0.3% Si, 0.3% Mn, 12% Cr, 0.8% Mo, and 0.8% V) by co-evaporation of iron and titanium. Subsequently this surface film was implanted with carbon at energies of 55, 120, and 200 keV to a total fluence of 1.24×1018C+ cm-2. This treatment produced a surface hardness of 15 GPa. The effect of this coating on unlubricated wear and friction was tested in air at a relative humidity of 10% in a pin-on-disc tester using a 440C pin as a counterface. The sliding mechanism of the untreated substrate was found to be based on the transfer of pin material and subsequent growth of uneven oxide hillocks on the wear track. Oxide scales were observed also on a wear scar of the pin, presumably as a result of back-deposition. In contrast, the sliding mechanism on the coated sample was drastically different. A more uniform transfer film originating in the coating was found on the pin, resulting in sliding between identical materials. No wearthrough of the coating occured during the test of 5000 cycles at a hertizian pressure of 835 MPa, and the surface of the wear track showed extreme smoothness to the very end of the test. The friction coefficient was decreased from 0.7 for the uncoated to 0.2 for the coated surface. The wear track on the coated surface was also found to be slightly oxidized, as determined by a nuclear reaction (16O(d,p)17O). The reduction in friction was mainly attributed to the increased hardness of the counterfaces and an adequate but controlled oxidation.

Migration of carbon in tempered martensitic steel during excimer laser melting

The migration of ion implanted {sup 13}C in tempered martensitic steel (nominal composition 1.05 wt. % C, 0.2 wt. % Si, and 0.3 wt. % Mn) during excimer laser melting was examined utilizing the resonance of the {sup 13}C(p,{gamma}){sup 14}N reaction at Ep = 1747.6 keV. Depth concentration profiles after five and ten laser pulses of 1 J/cm{sup 2} revealed deviation from random walk diffusion in a homogeneous media. This was modelled by using the solubility controlled flow of carbon in iron-carbon melt. A diffusion length of 2{radical}D{tau} 34 {plus minus} 2 nm during a period {tau} of the melted phase was deduced. Significant amorphous to crystalline transformation occurred during the rapid self quenching following laser melting. 18 refs., 3 figs.

EFFECT OF 690-KEV XE ION IRRADIATION ON THE MICROSTRUCTURE OF AMORPHOUS MOSI2/SIC NANOLAYER COMPOSITES

The effect of 690 keV Xe ion irradiation at three different dosage levels, 1, 5 and 10{times}10{sup 15}/cm{sup 2}, on the microstructure of amorphous-MoSi{sub 2}/amorphous-SiC nanolayer composites has been studied using transmission electron microscopy. Results show that the depth of radiation damage in this multilayer material is {approximately}80 nm, which agrees qualitatively well with the calculated damage depth calculated by TRIM. A diffraction ring corresponding to the (10{bar 1}1) plane of C40 MoSi{sub 2} was found in the electron diffraction pattern taken from the irradiated regions; the C40 phase is also found after thermal annealing of amorphous MoSi{sub 2} at 500{degrees}C or above. In the damaged regions SiC layers were found to spherodize while the nanocrystalline grains in the MoSi{sub 2} layers appeared to coarsen with increasing dose.