Papers by Author: Kung Jeng Ma

Abstract: As the trend of electronic industry is fast moving towards miniaturization, diversity, high efficiency and high throughput, the devices to be mounted onto a PCB are increasingly densely packed so are the holes needed to keep all elements in place. To effectively and economically generate so many holes of small diameter and high aspect ratio, the drilling process has to be done in a high speed manner and tool life has to be kept as long as possible. This study aimed to improve the tool life and the quality of the obtained holes by applying various hard coatings on the drills. The results showed that tool life could be effectively improved from 2500 hits to around 11000 hits at 155Krpm and 3.5m/min when proper hard coating was applied. It was found that wear of drill was resulted mainly from abrasive wear and adhesion wear. It was also showed in the research that while non-coated drills suffered serious deformation after 500 hits those coated with Al2O3 could finish 11000 hits with very limited wear land on the drills.

Abstract: Ceramic coatings have been widely used in cutting tools and various machine parts. Even though high strengths have been obtained in most ceramic coatings, it has also been shown that ceramic coatings undergo extensive plastic deformation during scratch and wear tests. Therefore, it is essential to understand the plastic flow and related friction and wear behaviour. Reciprocating multipass wear tests have been carried out on chemical vapor deposition (CVD) TiC coatings. Obvious plastic flow was observed on the rough surface of CVD TiC ceramic coatings in the first sliding, due to the extremely high contact pressure developed on the contact asperities. However, shake down may be quickly reached after several subsequent traverses. In further repeated traverses, the plastic-elastic flow accumulates residual strain energy to the point where cracking, microbuckling, and microflaking may occur along the elastic-plastic interfaces behind the indenter. The new rough surface will appear after the detachment of the heavily strained plate-like wear debris. The repeated sliding allows the process- “plastic flow of asperities - flatten the surface and shake down - microbuckling and detachment of strained layer” to continue until the coating is totally worn out.

Abstract: This research aimed to design and develop a polishing system for precision polishing mini roller mold to nanometer surface finish. An experimental polishing system was built in the present study to polish nickel plated specimens with various polishing compounds. The polished specimens were subsequently examined by Alfa-step, OM and SEM for surface finish, morphology and microscopic analysis respectively. The obtained surface condition and material removal rate were correlated to the polishing parameters such as spindle speed, abrasive concentration, and abrasive grit size for the improvement of the polishing effect. Mini-rollers of 5mm in diameter, 50mm in length were successfully polished to a surface roughness better than 2nm Ra in several hours without damaging the roundness and cylindricalness using abrasive of 0.3μm, 10,000rpm polishing speed and 0.5mm gap distance between polisher and the specimen. A semi-empirical model of polishing was also developed in the study for predicting the materials removal rate.

Abstract: There are growing varieties of glasses available on the market for the manufacture of molded optical lenses. A glass with a low transition temperature (Tg) has the advantage of extending the service life of molding dies. However, most of the low Tg glasses have a high content of alkali metal oxides and tend to induce severe glass sticking problems. This has made the molding process of these kinds of glasses very difficult indeed. The low Tg glasses normally demonstrate poor chemical durability and scratch resistance. As a result, the yields of fabricating the glass-preforms are frequently rather low. This research tried depositing a very thin layer of aluminum oxide on various glass-preforms by a water based sol-gel process. A high temperature glass wetting experiment was carried out to investigate the high temperature interfacial reaction between the coated glass gobs and stainless steel substrate. It was found that when the uncoated glass-preforms were brought into contact with stainless steel, the contact angle decreased with increasing heating temperature and duration. Owing to the severe interfacial chemical reaction, the originally transparent glass gradually turned translucent. In the case of Al2O3 coated glass-preforms, the variation of the contact angles was very limited, which presented no sticking and no wetting behavior. No reaction products could be detected on the contact area after the wetting test. The optical transmission of those lenses molded from the coated glass-preforms exhibited no or very little changes after the molding process.

Abstract: The glass molding process provides great potential for mass production of precise glass optical components at low cost. The key issue for achieving a low production cost is to extend the service life of the expensive mold inserts. The precious metal based alloy is one of the coating materials for the molds which provides excellent glass anti-sticking results. However, the inter-diffusion between the WC/Co mold materials and precious metal coatings will deteriorate the coatings which needs to be resolved. It is essentially to deposit an interlayer as the diffusion barrier to improve the inter-diffusion problem. A thin layer of TaN was deposited on the WC/Co substrate as the diffusion barrier using a magnetron sputtering system, and followed by the deposition of Pt-Ir layer as the protective layer. Low Tg Glass gobs (L-BAL 42) were placed on the coated substrate to investigate inter-diffusion between the substrate and coating at high temperature. The surface interaction between the glass gobs and protective coatings was also examined. The obtained TaN and Pt-Ir multilayer had a dense nano-crystalline structure. High temperature wetting tests showed that the TaN film could effectively resist the cobalt and tungsten diffusion into the precious metal protective layer and, as a result, minimized the possibility of oxidation and interaction between glass and protective coating. The coated substrates retained a good surface finish and the glass gobs stayed fully transparent after 6 hours reaction test at 700°C.

Abstract: The glass molding process (GMP) is regarded as a very promising technique for mass producing high precision optical components such as spherical/ aspheric glass lenses and free-form optics. However, only a handful of materials can sustain the chemical reaction, mechanical stress and temperature involved in the glass molding process. Besides, almost all of these mold materials are classified as hard-to-machine materials. This makes the machining of these materials to sub-micrometer form accuracy and nanometer surface finish a rather tough and expensive task. As a result, making mold life longer has become extremely critical in the GMP industry. The interfacial chemical reaction between optical glass and mold is normally the main reason for pre-matured mold failure. This research aimed to investigate the interfacial chemical reaction between various optical glasses, different anti-stick coating designs and several mold materials. The results showed that glass composition, coating design (composition, microstructure, thickness), environment (vacuum, air or in protective gas), reaction temperature and time could all have profound effects on the interfacial chemical reaction. Based on the results, a design developed specially for certain glasses is more likely to be the viable way of optimizing the effect of the protective coating.

Abstract: . Glass molding process is considered to have a great potential for the mass production of optical components with lower cost. Up to now, the service life of molding dies is still not satisfied. This study mainly focuses on the development of sol-gel derived Al2O3 coatings in order to extend the service life of glass molding dies. High temperature glass wetting experiment was carried out to investigate the high temperature interfacial reaction between the coatings and glass gobs. The 304 stainless steel were used as the substrate materials. The sol-gel coated Al2O3 was selected as the protective coating. OHARA L-BAL42 glass gobs were chosen as the test materials. It can be observed that very severe interfacial reaction occurs between stainless steel and glass at high temperature. The contact angle dramatically decreases from initial and then gradually approaches 25° at 825°C after 2 minutes holding time. The severe interface chemical reaction also results in the loss of transparency in glass appearance. The reaction products such as Zn and Ba are mainly from glass material. For the case of Al2O3 coated substrate, the variation of the contact angles varied from 152 to 138°, presents anti-sticking or non wetting behavior. There are no reaction products can be found on the glass contacted area after wetting test.

Abstract: Diamond is one of the most important engineering materials for its extreme hardness, high thermal conductivity value and chemical inertness. Due to its high hardness and strength, it can be ideal candidates for AFM probe or micro-needle. In this research, micro cone-like shaped diamond tips with high aspect ratio formed using reactive ion etching (RIE) method. The scanning electron microscope (SEM), transmission electron microscope (TEM) and micro-Raman spectroscopy were used to study the surface morphology and sub-surface micro-structure before and after RIE process. The results showed that gold could be adopted as mask material during the RIE process. Different microstructures could be obtained using different RIE parameters such as etch duration and reactant gas. After RIE (O2 50sccm, 200W) for 5min the micro cone-like structures (aspect ratio~8) could be observed on the surface if a thin layer of gold was applied as mask. However, under the same RIE conditions, the irregular pillar-like microstructures started to emerge if the etching time was stretched longer.

Abstract: The glass molding process is considered to have a great potential for the mass production of aspherical glass lenses with high precision and low cost. However, glass molding has a serious problem of mold sticking with glass which needs to be resolved. This research investigates the interface reaction between glass and mold by high temperature wetting experiment, which provides the reference for the designing anti-stick coatings. The SUMITA K-PSK200 optical glass gobs with low Tg were used in this study. The influence of operation temperature, ambient gas, substrate materials, and thin film composition on wettability of glass at high temperature were studied. The results show that the higher the temperature, the smaller the wetting angle between glass gob and substrate could be observed. This indicates that severe interface chemical reaction occured and resulted in the loss of transparency in glass appearance. The wetting experiment in nitrogen ambient improved the sticking situation. The combination of chemically stable substrates and coatings, such as Sapphire (substrate) / GaN (film) and Glass (substrate) / Al2O3 (film) can achieve the best antistick propose. The precious metal films, such as Pt, Ir, coated on the ceramic substrates can effectively reduce the interface reaction between the glass and substrates.

Abstract: Stainless steel and other ferrous metals are normally classified as not diamond turnable for the unacceptable tool wear caused by the thermal-chemical reaction between diamond and ferrous metals. In the present research, stainless steel specimens were plasma nitrided at a relatively low temperature (<450oC) to prevent the depletion of Cr content at the austenite matrix and to give a hardened layer where Fe atoms are bonded to nitrogen atom to form γ’-Fe4N. Diamond turning experiments were subsequently carried out under the following machining conditions: single crystal diamond tool, cutting speed up to 180 m/min, cut depth up to 5μm and light mineral oil as the cutting fluid. The results showed that, given the same machining conditions, while rapid tool wear and poor surface finish were obtained when turning the as-received stainless steel, surfaces with Ra better than 3nm and no obvious tool wear were achieved when turning the plasma nitrided specimens