Effect of Substrate Temperature on Deposition Behavior of Copper Particles on Substrate Surfaces in the Cold Spray Process

The deposition behavior of sprayed individual metallic particles on the substrate surface in the cold spray process was fundamentally investigated. As a preliminary experiment, pure copper (Cu) particles were sprayed on mirror-polished stainless steel and aluminum (Al) alloy substrate surfaces. Process parameters that changed systematically were particle diameter, working gas, gas pressure, gas temperature, and substrate temperature, and the effect of these parameters on the flattening or adhesive behavior of an individual particle was precisely investigated. Deposition ratio on the substrate surface was also evaluated using these parameters. From the results obtained, it was quite noticeable that the higher substrate temperature brought about a higher deposition rate of Cu particles, even under the condition where particles were kept at room temperature. This tendency was promoted more effectively using helium instead of air or nitrogen as a working gas. Both higher velocity and temperature of the particles sprayed are the necessary conditions for the higher deposition ratio in the cold spraying. However, instead of particle heating, substrate heating may bring about the equivalent effect for particle deposition. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

倾斜入射条件下衬底对金属网栅屏蔽特性的影响

提出了双层膜系分析方法,该方法采用独立网栅等效薄膜模型,将网栅对衬底干涉的影响引入分析,克服了Kohin方法仅考虑衬底干涉的不精确性,并避免了其分析独立网栅的透波率公式在高频和大入射角时存在的不准确性。用紫外光刻在石英衬底上制备了周期为320 μm,线宽为4.5 μm的网栅,测得S波30°入射时12~18 GHz的屏蔽效率大于16 dB,略低于理论值但变化趋势一致,证实了双层膜法的有效性。进而分析衬底影响表明:入射为S波及小于衬底布儒斯特角θ_B的P波时屏蔽效率降低,而入射为大于θ_B的P波时屏蔽效率反常增加;屏蔽效率变化最大值随入射角变化;优化衬底厚度可在不影响网栅透光能力的同时获得最佳屏蔽效果。     

蓝宝石基片的磁流变化学抛光试验研究

分析了磁流变化学抛光的加工机理,对蓝宝石基片的磁流变化学抛光进行了试验研究。结果表明:磁流变化学抛光可将蓝宝石基片加工到亚纳米级粗糙度的超光滑镜面,材料去除率受化学反应速率和剪切去除作用共同影响,化学反应速率越快,剪切去除作用越强,材料去除率越高;混有硅胶溶液与α-Al2O3磨料的磁流变化学抛光液去除率最高;材料去除率随工作间隙,励磁间隙以及加工时间的增大而减少,随铁粉浓度减少而减少;利用磁流变化学抛光方法加工蓝宝石基片可获得Ra 0.3 nm的超光滑表面。     

Progress in 4H-SiC homoepitaxial growth by hot-wall LPCVD

Recent progress in 4H-SiC homoepitaxial growth by using hot-wall low-pressure chemical vapor deposition (HW-LPCVD) at the authors' group was reviewed. Horizontal air-cooled low-pressure hot-wall CVD (LPHWCVD) system was developed and employed to get high quality 4H-SiC epilayers. Homoepitaxial growth of 4HSiC on off-oriented Si-face (0001) substrates was performed at a typical temperature of 1 500 ℃ by using the stepcontrolled epitaxy. The typical growth rate was controlled to be about 1.0 μm/h. The background doping was reduced to 2 × 1015- 5 × 1015 cm-3. The FWHM of the rocking curve was 0 308 nm in 2 μm× 2 μm scale. Intentional N-doped and B-doped 4H-SiC epilayers were obtained by in-situ doping of NH3 and B2 H6, respectively. Thermal oxidization of 4H-SiC homoepitaxial layers was conducted in a dry O2 and H2 atmosphere at 1 150 ℃ and 4H-SiC MOS structures was obtained. Schottky barrier diodes with reverse blocking voltage of over 1000 V were achieved preliminarily.     

Effect of substrate temperature on adhesion strength of plasma-sprayed nickel coatings

We plasma-sprayed nickel coatings on stainless steel and cobalt alloy coupons heated to temperatures ranging from room temperature to 650 °C. Temperatures, velocities, and sizes of spray particles were recorded while in-flight and held constant during experiments. We measured coating adhesion strength and porosity, photographed coating microstructure, and determined thickness and composition of surface oxide layers on heated substrates. Coating adhesion strength on stainless steel coupons increased from 10–74 MPa when substrate temperatures were raised from 25–650 °C. Coating porosity was lower on high-temperature surfaces. Surface oxide layers grew thicker when substrates were heated, but oxidation alone could not account for the increase in coating adhesion strength. When a coupon was heated to 650 °C and allowed to cool before plasma-spraying, its coating adhesion strength was much less than that of a coating deposited on a surface maintained at 650 °C. Cobalt alloy coupons, which oxidize much less than stainless steel when heated, also showed improved coating adhesion when heated. Heating the substrate removes surface moisture and other volatile contaminants, delays solidification of droplets so that they can better penetrate surface cavities, and promotes diffusion between the coating and substrate. All of these mechanisms enhance coating adhesion.     

Influence of dc bias on amorphous carbon deposited by pulse laser ablation

Amorphous carbon films were deposited on single-crystalline silicon and K9 glass by pulse laser ablation using different negative substrate bias.Scanning electron microscope(SEM) was used to observe morphology of the surface.Thickness and refractive index of the film deposited on K9 glass were measured by ellipsometry.Micro-hardness of films was measured relatively to single crystal silicon.All films deosited on silicon were analyzed by Raman spectra.All spectra were deconvoluted to three peaks.Line-width ratios varied similarly with bias voltage when the laser energy was kept invariant.     

Splat morphology and microstructure of plasma sprayed cast iron with different preheat substrate temperatures

A cast iron coating is a prime candidate for the surface modification of aluminum alloys for antiwear applications because cast iron is inexpensive and exhibits superior wear resistance arising from the self-lubricating properties of graphite. In the present study, fundamental aspects of a plasma sprayed cast iron coating on an aluminum alloy substrate, including (1) the effects of preheat substrate temperature on the splat morphology, (2) the formation of a reaction layer and pores, and (3) the splat microstructure, were investigated in low-pressure plasma spraying. With an increasing substrate temperature, the splat morphology changes from a splash type to a disk and star shape. Deformed substrate ridges mainly resulting from the slight surface melting, are recognized adjacent to the splat periphery at high substrate temperatures. The flattening ratio of disk splats decreases with substrate temperature because the ridges act as an obstacle for splat expansion. A reaction layer composed of iron, aluminum, and oxygen is ready to form at high substrate temperatures, which, along with the deformed ridges, improves the adhesive strength of splats. However, the pores appear at the splat interface at low substrate temperatures, which hinder the formation of a reaction layer. The amount of graphitized carbon increases in cast iron splats with an increase in substrate temperature.     

钢基体涂覆Al2O3陶瓷层的研究现状与展望

综述了目前钢基体上制备Al2O3陶瓷涂层的研究现状，详细介绍了热喷涂法、溶胶一凝胶法及热化学反应法3种制备Al2O3陶瓷涂层的主要方法，并探讨和分析了它们各自的优缺点。热喷涂法普遍存在维护和使用费用昂贵的特点；溶胶一凝胶法最大优点就是制备过程温度低；热化学反应法作为一种新兴的钢基Al2O3陶瓷涂层制备方法，其显著优点在于工艺简单、成本低廉并且无需特殊设备，具有广阔的发展前景。文章最后对Al2O3陶瓷涂层的研究进行了展望。     

Solidification modeling of plasma sprayed TBC: Analysis of remelt and multiple length scales of rough substrates

A two-dimensional, finite-element model based on an enthalpy formulation, was developed to simulate a splat solidifying on a rough substrate (with an idealized, sinusoidal-shaped roughness) capturing the multiple-length scales seen in real coatings as well as different aspect ratios. The model was used to study the effects of substrate temperature, splat temperature, and roughness characteristics on the onset and extent of remelt. Remelt is studied since it is indicative of local heat transfer conditions and might explain observed coating properties. Multiple splats were simulated using the two-dimensional model for short-time cooling coupled to a one-dimensional model for long-time cooling to predict substrate temperature rise prior to subsequent splat impacts. The presence of roughness promoted substrate remelting at conditions under which no remelting was observed for a smooth surface, suggesting that substrate roughness is an important parameter to include in splat solidification studies. The effects of splat temperature and substrate temperature on remelt were consolidated into a single nondimensional parameter, which captured a number of critical phenomena including characterization of the onset of remelt with a nondimensional remelting point.     

Smart Substrate MCMs

This paper presents a cost-based assessment of the effectiveness of Smart Substrate MCM Systems. A Smart Substrate MCM System is one in which the substrate contains active circuitry for carrying out testing functions. The feasibility of using this approach is investigated. The Smart Substrate strategy is compared to an alternative approach based on the assumption that system components are perfect (Known Good Die (KGD) approach). The obtained results identify the domain of applicability of Smart substrate MCMs and point to limitations of the KGD approach.