Improving the Wear Behavior of WC-CoCr-based HVOF Coating by Surface Grinding

WC-CoCr-based high velocity oxy fuel (HVOF) coatings are being used for several components which are prone to severe erosion or abrasion. In this study, the HVOF coating was applied by liquid fuel-based equipment. These coated samples were subjected to surface grinding of various depths (100, 200, and 300 μm). Hardness test after surface grinding showed that the coating hardness increased by 33% after grinding to a depth of 200 μm (1472 Hv). The residual stress after different depths of grinding was measured using x-ray diffraction. It showed that the compressive residual stress of coating increased with grinding. Increase in hardness of the coating (after grinding) is believed to be due to the increase in compressive residual stress. The abrasive wear resistance increased after grinding to a depth of 100 μm thickness and remained constant during successive grinding. In contrast, the erosive wear resistance increased the most when the grinding thickness was 200 μm. It is concluded that the surface grinding of coatings helps in increasing abrasive and erosive wear resistance. The increase in microhardness of the coating is believed to be the reason for high wear resistance. SEM studies of worn out surface show carbide grain pull out due to removal of softer phase, i.e. cobalt and chromium, and is followed by tungsten carbide grain pull out.     

Investigations of Solidification Structures of High Carbon Alloy Cast Steel Containing RE-V-Ti

The solidification microstructural characteristics of rare earth (RE) and vanadium and titanium-treated high carbon alloy cast steel, whose chemical compositions are 0.90 wt.% C, 3.0 wt.% Si, and 1.0 wt.% Mn, were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), electron probe microanalyzer (EPMA), and energy dispersive X-ray spectrometry (EDS). The alloys were prepared in an induction furnace. The results showed that compound additions of RE-V-Ti could obviously refine the solidification structures and decrease the diameter of grains. The average grain diameter of untreated high carbon alloy cast steel was 80 μm. The average grain diameter of high carbon alloy cast steel containing RE-V-Ti decreased to 40 μm. Finally, the reasons that RE-V-Ti refined the solidification structure were analyzed in detail.     

Manufacture and properties of cold spray deposited large thickness Cu coating material for sputtering target

In this study, the manufacture of a large thickness Cu coating layer as sputtering target material via a cold spray coating process was undertaken. The microstructure and properties of the Cu layer as the sputtering target material (before and after the annealing heat treatment) were evaluated, compared, and analyzed. To evaluate the purity, density, grain size and uniformity, microstructure, and properties of the Cu-coated layer as a sputtering target, X-ray diffraction, ICP analysis, SEM, EBSD, porosity analysis, and Vickers hardness tests were performed. The result of the observation of the layer’s purity and microstructure showed that a purity level (99.47%) similar to that of the early powder 2N5 was maintained and that the manufacture of a cold spray deposited, ∼20 mm thick Cu coating material for the sputtering target was performed successfully. As a result of the EBSD mapping, the average grain size near the interface and around the center measured 1.48 μm and 1.49 μm; the grains were small and non-uniform compared with the 1.91 μm size near the surface. Note, however, that the recrystallization and grain growth (caused by annealing) increased the grain size to 1.82 μm (near the interface), 1.83 μm (near the center), and 1.87 μm (near the surface) and improved the level of uniformity. Moreover, through post heat treatment, the overall porosity declined (0.44 % porosity/400 °C/h heat treatment), and the grain texture became uniform. The possibility of controlling the microstructure as a large thickness sputtering target by conducting an annealing heat treatment was also confirmed. Nonetheless, the differences in the porosity and hardness associated with the coating thickness changes were partially maintained. Based on the aforementioned findings, this study suggests that by using cold spray deposition, Cu coating layers with large thicknesses can be applied as a sputtering target.     

The effect of submicron-sized initial tungsten powders on microstructure and properties of infiltrated W-25 wt.% Cu alloys

In the present work, several W-25 wt% Cu alloys have been prepared through combined processes of high-energy ball-milling, liquid-phase sintering and infiltration, using the precursors of industrial copper powders with an average particle size of 50 μm and tungsten powders with alternative average particle size of 8 μm, 800 nm, 600 nm or 400 nm. Microstructure characteristics, relative density, hardness and electrical conductivity of the WCu alloys were investigated to elucidate the effect of initial particle size of tungsten powders. EBSD was further utilized to reveal the orientation and grain size distribution in the WCu alloys prepared by 8 μm and 400 nm-sized tungsten powders. The results showed that the WCu alloy made by 400 nm-sized tungsten powders exhibited excellent homogeneity for both sintered tungsten powders and grains, together with the highest relative density of 98.9%, the highest hardness of 230 HB, and good electrical conductivity of 48.7% IACS. Moreover, it also showed highly improved arc erosion and mechanical wear resistances.     

Particle size versus grain size relation in the sprayformed Al-18Si composites

Various hypereutectic Al-18Si-X composites with different average Si particle sizes ranging from 0.2∼30 μm were produced by varying process parameters of spray forming. The effect of the Si particle size in determining the matrix grain size was investigated in order to design the optimum process for producing hypereutectic Al-18Si-X composites with fine microstructures. The experimental results suggested that the Si particle size greater than 2 μm is desirable for retarding the grain coarsening during subsequent forming processes, resulting in the fine microstructure. The observed results on the coarsening behavior of grains during spray-forming were analyzed from the free energy viewpoint.     

Characterization of irradiated simulated DUPIC fuel

A simulated DUPIC (Direct Use of Spent PWR Fuel in CANDU Reactors) fuel was irradiated at HANARO research reactor of KAERI in 1999. Post-irradiation examinations, such as measurements of γ-scanning, profilometry, density, hardness, microstructure, and fission product distribution were performed on the irradiated simulated DUPIC fuel. In γ-scanning, the intensity along the axial direction was sharply decreased at the areas between the pellets. There was no significant change in the profilometry of SEU-1.47%, but variation was detected in SEU-2.19%+F.P by 67 μm, and the peaks precisely coincided with the ridges of the pellets. The marked difference between SEU-1.47% and SEU-2.19%+F.P pellets after irradiation was the configuration of cracks arised in the pellets. Some large equiaxed grains of 11.1 μm were observed at the center of the SEU-2.19%+F.P pellet, while the grain size near the surface of the pellet was remained almost the same as the original grain size of 5.58 μm. The hardness had no tendency toward change to the direction, but average hardness was increased as much as 10% compared with a fresh simulated DUPIC fuel. This article is based on a presentation made in the “Symposium on Nuclear Materials and Fuel 2000”, held at the Korea Atomic Energy Research Institute (KAERI), Taejon, Korea, August 24–25 under the auspices of the Ministry of Science and Technology (MOST).     

Cavitation Erosion Resistance of Fe-Based Amorphous/Nanocrystal Coatings Prepared by High-Velocity Arc Spraying

Two kinds of amorphous/nanocrystal coating (FeCrNiBSiNb and FeCrBSiWNb) were prepared by high-velocity arc spraying. The microhardness, bonding strength, and cavitation erosion resistance of the coatings were measured. The microstructure, amorphous content, and nanocrystal size were checked and the surface morphologies were investigated after cavitation erosion testing. The results indicated that both the FeCrNiBSiNb and FeCrBSiWNb coating consisted of amorphous/nanocrystal and Fe-based solid solution, with amorphous content of 73.6 and 57.2 vol.%, respectively. The Ni-Cr-Fe solid solution of the FeCrNiBSiNb coating had an average grain size of 19.8 nm, whereas the Fe-Cr solid solution of the FeCrBSiWNb coating had an average grain size of 29.4 nm. Moreover, both the FeCrNiBSiNb and FeCrBSiWNb coating exhibited good bonding strength, high hardness, and excellent cavitation erosion resistance. After 180 min of cavitation erosion, the cumulative weight loss of the FeCrNiBSiNb and FeCrBSiWNb coating was 21.3 and 24.0 mg, whereas the weight loss of a 0Cr18Ni9 coating was up to 62.6 mg. This investigation revealed that the cavitation damage to the thermally sprayed amorphous/nanocrystal coatings was mainly in the form of layer detachment, whereas for the conventional coating it took the form of particle breakage.     

Growth of AlN Films and Its Process Development for the Fabrication of Acoustic Devices and Micromachined Structures

AlN films were grown on silicon substrates by RF reactive magnetron sputtering. At high sputtering powers, (002) preferred orientation as well as Al-N absorption band becomes prominent. The surface roughness and grain size of sputtered films were found to increase with RF power. Surface acoustic wave (SAW) device has been made on the grown (002) oriented piezoelectric AlN film with interdigital transducer (IDT) electrodes spacing corresponding to a wavelength of 60 μm. The centre frequency of the SAW filter was found to be 84.304 MHz, which gives a phase velocity of 5058 m/s with an electromechanical coupling coefficient (K ²) of 0.34%. Low etch rate of AlN films were observed in doped TMAH solution. Three-dimensional suspended Cr/AlN/Cr/SiO₂ microstructures were also fabricated by wet chemical etching.     

Microstructure evolution and low temperature superplasticity of ZK40 magnesium alloy subjected to ECAP

Microstructure evolution and superplastic behaviors of ZK40 magnesium alloy were investigated in the temperature range of 473–623 K. Transmission electron microscopy (TEM) was used to study the microstructure changes. After the alloy had been processed by equal channel angular pressing (ECAP) for one pass through the die, significant twinning was found to have occurred, and the mean grain size was 5.6 μm. Finer grains were obtained after multi-pass ECAP, and the average grain size of the alloy ECAPed for three passes was as low as 0.8 μm; this alloy exhibited low temperature superplasticity at 473–523 K, and the elongations obtained at the initial strain rate of 1×10⁻³ s⁻¹ were 260% at 473 K and 612% at 523 K. Corresponding values for the ZK40 alloy processed by ECAP for only one pass were 124% at 473 K and 212% at 523 K. Poor superplastic behavior of the ZK40 alloy processed by ECAP for only one pass was related to the longrange stresses associated with the non-equilibrium grain boundaries within the coarse grains. The incompatibility between the fine grains and the coarse grains was thought to be unfavorable to the improvement of superplascity. This article is based on a presentation in “The 7th Korea-China Workshop on Advanced Materials” organized by the Korea-China Advanced Materials Cooperation Center and the China-Korea Advanced Materials Cooperation Center, held at Ramada Plaza Jeju Hotel, Jeju Island, Korea on August 24–27, 2003.     

A Study on Microstructure and Dielectric Performances of Alumina/Copper Composites by Plasma Spray Coating

In this study, surfaces of copper plates were coated with a thick alumina layer by the plasma spray coating to fabricate a composite with a dielectric performance that made them suitable as substrates in electronic devices with high thermal dissipation. The performance of alumina dielectric layer fabricated by the plasma spray coating and traditional screen-printing process was compared, respectively. Effects of the spraying parameters and size of alumina particles on the microstructure, thickness, and the surface roughness of the coated layer were explored. In addition, the thermal resistance perpendicular to the interface of copper and alumina and the breakdown voltage across the alumina layer of the composite were also investigated. Experimental results indicated that alumina particles with 5-22 μm in diameter tended to form a thicker layer with a poorer surface roughness than that of the particles with 22-45 μm in diameter. The thermal resistance increased with the surface roughness of the alumina layer, and the breakdown voltage was affected by the ambient moisture, the microstructure and the thickness of the layer. The optimal parameters for plasma spray coating were an alumina powder of particles size between 22 and 45 μm, a plasma power of 40 kW, a spraying velocity of 750 m/s, an argon flow rate of 45 L/min, a spraying distance of 140 mm, and a spraying angle of 90°. It can be concluded that an alumina layer thickness of 20 μm provided a low surface roughness, low thermal resistance, and highly reliable breakdown voltage (38 V/μm).     

Fabrication of High Performance PM Nanocrystalline Bulk AA2124

AA2124 nanopowders <100 nm in particle size and 20 nm internal structure produced by high energy ball milling of gas-atomized micronpowders ~45 μm in particle size and 700 nm internal structure were processed in to bulk rods. The micro- and nanopowders were hot compacted using uniaxial pressing for preliminary densification at 0.7T _m of the alloy. Selected intact hot compacts (HCs) were promoted for warm severe plastic deformation via equal channel angular pressing (ECAP) at the minimum possible deforming temperature for final densification. Effect of the fabrication method of the consolidated powders was investigated. A combined processing via HC/ECAP produced bulk nanostructured rods 2.5 μm and 50-60 nm in grain size for the micro- and nanopowder consolidates, respectively. The powder properties controlled the degree of densification and mechanical behavior during the hot compaction stage, which influenced strongly the deformation behavior during subsequent ECAP. At the end of HC/ECAP one pass, the HC stage was responsible for about 83 and 95% of the total grain coarsening encountered for the micro- and nanopowder HCs, respectively. Throughout the various consolidation stages employed, the ball-milled (BM) nanopowder consolidates exhibited 2/3 the grain growth and displayed almost twice the hardness and compressive strength values of the gas-atomized micronpowder ones. Influence of BM and HC on the Al₂O₃ layer formed around the individual powder particles was also investigated.     

Effect of hot rolled grain size on the precipitation kinetics of nitrides in low carbon Al-killed steel

The precipitation of nitrides plays a general role in the industrial processing of deep drawing quality Al-killed low carbon steels. In this paper, the effect of hot rolled grain size on the precipitation of nitrides has been analysed. To evaluate the effect of grain size on the nitride precipitation kinetics, thermoelectric power based investigations have been performed on hot and cold rolled specimens.In the hot rolled state, the precipitation of nitrides occurs more intensively in the fine grain size microstructure (average grain size = 9 μm) than in the large grain size microstructure (average grain size = 23 μm) until the precipitated fraction of nitrides reaches about 70%. In the cold rolled state the effect of grain size is much less significant; probably the precipitation process occurs simultaneously at the grain boundaries and along dislocations. According to the simulation results, significant differences can be found between the precipitated fraction of nitrides in fine and large grain size sheets coiled in the temperature range 550–650 °C. In this interval, the precipitated nitride fraction is about two times larger in a fine grain microstructure (9 μm) than in sheets with 23 μm average grain size. The local position in the coil also affects significantly the precipitated fraction of nitrides. In the outer ring of the coil, less than 20% precipitated fraction is predicted in coiling temperature range 550–700 °C. However, in the middle ring of a hot rolled coil, the precipitated fraction changes from 5% to 85% with increasing coiling temperature from 550 to 700 °C.     

Effect of initial α-phase content on microstructure and flexural strength of macroporous silicon carbide ceramics

The effects of the initial α-phase content on the microstructure and flexural strength of macroporous silicon carbide (SiC) ceramics were investigated. When β powder or a mixture of α/β powders containing small (≤3%) amounts of α powder were used, the grains showed a platelet-shape. In contrast, the grains had an equiaxed-shape when α powder or a mixture of α/β powders containing large (≥50%) amounts of α powder was used. The flexural strength increased with increasing α-SiC content in the starting composition, whereas the porosity decreased with increasing α-SiC content. The strength of the macroporous SiC ceramics was affected mostly by the porosity when the grain size was smaller than 10 μm, whereas the strength was controlled by pore size and grain size when the microstructure consisted of large (>10 μm) platelet grains.     

Effect of grain size on creep properties of type 316LN stainless steel

The effect of grain size on creep properties of type 316LN stainless steel has been investigated at 600°C under different stresses. The initial strain at the beginning of creep tests decreased with the decrease of grain size. This was confirmed by the Hall-Petch relationship. The steady state creep rate decreased to a minimum value at the intermediate grain size (dm=80–130 μm) and then increased with the further increase of grain size. This result agreed with Garofalo's model stating that grain boundaries act simultaneously as both dislocation sources and barriers to dislocation movement. The rupture elongation at the intermediate grain size was minimal due to the cavity formed easily by carbide precipitates in the grain boundaries.     

Aluminum Impurity Diffusion in Magnesium

The diffusion of Al in polycrystalline Mg (99.9%) was studied via depth profiling with secondary ion mass spectrometry in the temperature range of 573-673 K, utilizing the thin film method and thin film solution to the diffusion equation. Multiple samples with multiple depth profiles on each sample were obtained to determine statistically confident coefficients with a maximum standard deviation between measurements of 16%. The activation energy and pre-exponential factor of Al impurity diffusion in Mg were determined as 155 kJ/mole and 3.9 × 10⁻³ m²/s, respectively. The Mg substrates have a small grain size (~10 μm) and therefore some contributions from grain boundary diffusion are expected in the measurements. Sputter roughening during depth profiling, which is inherent to the SIMS process, also contributes to the measured diffusion coefficient, especially in samples with smaller grain sizes.     

Printing gold nanoparticles with an electrohydrodynamic direct-write device

A dispersion containing 15nm gold particles in ethanol was subjected to electrohydrodynamic atomization, producing droplets with an average size of ≈26μm as measured by laser diffraction. An electrohydrodynamic direct-write device was used to deposit the dispersion onto silicon wafers with the intention of printing gold tracks. Immediately after deposition ethanol evaporated, leaving the gold particles organised in a ≈75μm wide central region surrounded by two regions, each ≈50μm wide, at the edge of which two further gold tracks, each ≈7μm existed.     

The effects of cold rolling and heat treatment on Al 6063 reinforced with silicon carbide granules

The effects of cold rolling and heat treatment on the strength and ductility of aluminum alloy 6063 reinforced with silicon carbide granules have been examined. Silicon carbide (SiCp) 100 μm grain size was added to 6063 aluminum in volume fractions of 0–30% to produce samples for heat treatment and cold rolling. The results show that an optimum combination of strength and ductility at 137.92 MPa and true strain of 0.173 is achievable with rolled-and-tempered samples containing 10% SiCp. This is a significant improvement on 6063 aluminum alloy having an ultimate tensile strength of ∼100 MPa at true strain of 0.18.     

Effects of Arc Spray Process Parameters on Corrosion Resistance of Ti Coatings

In order to improve the corrosion resistance of carbon steel structures in marine environment, Ti wires were sprayed to a steel substrate using arc spray technique, and orthogonal experimental design was used to investigate the effects of the fluctuation in the main parameters of spray process on the microstructure and the corrosion resistance of sprayed coatings. The results show that the corrosion resistance of sprayed coatings is very sensitive to spray process parameters, corrosion current density can decrease from 997.7 to 5.08 μA cm⁻² by optimizing process parameters. The coatings are composed of TiN and Ti₂O, and the corrosion resistance of coatings can be improved with the decrease in the contents of oxides. The spray distance should be exactly monitored and controlled in arc spray process because of its great effects on the quality of sprayed coatings.     

Arc erosion behavior of a nanocomposite W-Cu electrical contact material

The erosion behavior of a nanocomposite W-Cu material under arc breakdown was investigated. The arc erosion rates of the material were determined, and the eroded surfaces and arc erosion mechanisms were studied by scanning electron microscopy. It is concluded that the nanocomposite W-Cu electrical contact material shows a characteristic of spreading arcs. The arc breakdown of a commercially used W-Cu alloy was limited in a few areas, and its average arc erosion rate is twice as large as that of the former. Furthermore, it is also proved that the arc extinction ability and arc stability of the nanocomposite W-Cu material are excellent, and melting is the major failure modality in the make-and-break operation of arcs.     

Effect of ultrasonic power on grain refinement and purification processing of AZ80 alloy by ultrasonic treatment

Ultrasound with different powers was applied to treat AZ80 alloy melt to attain grain refinement or purification processing of the alloy. The influence of ultrasonic powers from 0 W to 1400 W on microstructures of the AZ80 alloy with ultrasonic grain refinement treatment was investigated. The average grain size of the alloy could be decreased from 387 μm to 147 μm after the ultrasound with the optimal power 600 W was applied to treat the melt. The effect of ultrasonic powers from 0 W to 230 W on the inclusion distribution in the ingot that was treated by ultrasonic purification processing was also studied. The optimal ultrasonic power in the ultrasonic purification processing was 80 W. In order to gain insight into the mechanism by which ultrasonic power affected the microstructure of the alloy or inclusion distribution in the ingot, numerical simulations were carried out and the ultrasonic field propagation in the melt was characterized.