Papers by Keyword: Sapphire

Abstract: Sapphire is an attractive engineering material for the cover glass of smartphones and wristwatches because of its high hardness and resistance to corrosion, wear, and heat. However, the high rate of tool wear and brittle chippings around the holes are serious problems associated with drilling sapphire. To enhance the accuracy and efficiency of the drilling process on sapphire, this paper presents a novel tool-path strategy using helical milling along with various tool path patterns. Experimental results show that the proposed method successfully reduces brittle chippings around the holes.

Abstract: In the telescopic interferometer KAGRA (Large-scale Cryogenic Gravitational wave Telescope), the ultraprecision mirror made of sapphire (α-alumina) is required to polish. The focal length of the mirror is 7 km and it must be finished by polishing after grinding. In order to polish the mirror precisely, polishing characteristics of the sapphire must be researched. In this study, the polishing characteristics are examined and the dependence of the crystal orientation on the polishing rates is studied. In the experiments, sapphire wafer is polished by using rotational small tool and SiO₂ abrasives.

Abstract: Sapphire was improved by filling lithium-cobalt glass. Cobalt concentration in lithium glass is 0.1-2 wt%. The sapphires with glass were heated by an electric furnace at 1100 °C for 1-5 hours. Physical properties of the stone before and after treatment were investigated by measuring refractive index and specific gravity. Stones were investigated the inclusion and fissure by a gem microscope. Reflectance spectrum was analyzed by UV-visible spectrophotometer and color changes were referenced by CIEL*a*b* system. Fluorescence was examined with a UV Lamp. Experimental results of heated sapphire treatment with lithium-cobalt glass found that the refractive index (RI) decreased from 1.76 - 1.77 to 1.53 - 1.55 and the specific gravity (SG) decreased from 3.95 - 4.00 to 3.37 – 3.40. Cause of these results is that the refractive index and specific gravity of lithium-cobalt glass are lower than of sapphires. The cobalt in glass filled in fissure of sapphires is cause of blue color appeared in crack. The orange-red fluorescence appeared in both short wave and long wave of UV. For analysis of trace elements by the UV-visible spectrophotometer range 300-800 nm, cobalt peak appeared at wavelength 530 nm while iron peak and another peak of improved sapphire in all conditions were found at wavelength 488 nm and 750 nm, respectively. The height of peak at 750 nm depended on the amount of cobalt. The color of improved sapphire was closed to blue color when compared with sapphire before improvement.

Abstract: The production cost of the sapphire substrate was restricted by the efficiency of processing and the surface quality. A proposed level orthogonal experiment was conducted to reveal the effect of workbench speed, lapping pressure, the Concentration of triethanolamine and abrasive pad types on the material removal rate and surface roughness and morphology of sapphire substrate when lapped with a diamond fixed-abrasive pad. The results showed that the average material removal rate of sapphire is about 24μm/min, and the surface roughness Ra achieves 0.36μm when the 200/230 mesh diamond fixed-abrasive pad was used. The material removal rate and the surface roughness as the optimization goal, the optimal lapping parameters were as follows: the lapping pad with raised, lapping pressure 0.075MPa, workbench speed 120rpm and the Concentration of triethanolamine 1%. Under these optimal machining parameters, the material removal rate reached 42μm /min and the surface roughness Ra reached 0.37μm.

Abstract: This work examined improvements in cutting efficiency obtained during the slicing of sapphire workpieces when adding CeO₂ to the cutting fluid. Various machining parameters, including the cutting temperature, machining surface tolerance, surface quality and tool wear, were assessed. It was confirmed that both the cutting temperature and the machining surface tolerance are reduced by the presence of CeO₂ in the cutting fluid. Observations of the machined surfaces and the cutting tool also demonstrated that both brittle fracturing of the surface and loss of abrasive grains were suppressed following the addition of CeO₂. These results suggest that the cutting temperature likely affects the accuracy of the cutting process and that CeO₂ appears to improve cutting efficiency by reducing the workpiece temperature and by stabilizing the cutting process.

Abstract: A sapphire substrate is essential for epitaxial growth of GaN, which is used for high brightness light-emitting diodes (LEDs), high-power and high-frequency devices. However, the material removal rate (MRR) of sapphire in conventional polishing is very low because of its high hardness and chemical inertness. We proposed application of plasma assisted polishing using a resin-bonded silica grinding stone for finishing of sapphire surface and investigated basic removal properties. The results of a ball-on-disc type test results showed that irradiation of water vapor containing atmospheric pressure Ar gas plasma promoted the MRR of sapphire by a factor of 7.4. Strong emission from hydroxyl radical was observed by an optical emission spectroscopy measurement of the plasma. XPS measurements revealed that the surface of both sapphire and silica were hydroxylated after the plasma irradiation. From these experimental and measurement results, we proposed the removal model in plasma assisted polishing of sapphire as follows. Firstly, irradiation of water vapor plasma hydroxylates the surfaces of sapphire and silica. Then, Al-O-Si bonding is formed by dehydration reaction between sapphire and silica surfaces. Finally, surface atom of sapphire is removed by the motion of silica abrasive. In this paper, we describe the preliminary experimental results and measurement results which support the proposed removal model in plasma assisted polishing of sapphire.

Abstract: A production-scale multiwire saw machine and 4 inch sapphire ingots were used in this study. The diamond wire used in the study had a core diameter of 0.1mm with an attached diamond particle size of 8–12μm. This study uses the Taguchi method and Grey relational analysis on the key diamond wire parameters which are electroplated nickel layer thickness, diamond wire tension, diamond wire speed and sapphire ingot feed rate, in order to simultaneously optimize the cutting performance in the diamond wire sawing of sapphire ingots. Based on the analysis, the nickel layer thickness and wire speed are the first and second most significant factors with 31.7 and 29.9% effects on cutting performances. The optimal control factors were then simultaneously evaluated for Ra, material removal rate, diamond wire wear rate and TTV and were found at optimization to be 14 μm nickel layer thickness, 15NT wire tension, 800m/min wire speed and 0.2mm/min feed rate, respectively. Compared with current standard condition, this improved process obtained from the optimization of diamond wire electroplated nickel layer thickness and saw machine parameters in the diamond wire sawing of sapphire ingots can achieve a 33% lower Ra, a 20% lower diamond wear rate, a 13% lower TTV and a 20% higher material removal rate, simultaneously.

Abstract: We realized the growth of 3C-SiC crystal on sapphire by solution growth method. The carbon deposition on a sapphire substrate before growth is the key point for this technology. This carbon layer plays a role to protect the dissolution of sapphire by Si solvent. Single crystal of 3C-SiC was grown on the whole surface of the sapphire substrate. Surprisingly, the 3C-SiC layer did not directly grown on the sapphire substrate. The single crystal 3C-SiC layer formed by the reaction between the deposited carbon and the Si wafer that is a solvent material below the melting point of silicon during heating process before the growth. The 3C-SiC grew on the 3C-SiC layer. In this process, the deposited carbon play another important role.

Abstract: A pyrometer experiment was performed using the dynamic high pressure method; it shows that obvious emission phenomenon had been generated in the shocked sapphire windows. Shock stress in the sapphire was 1.3 Mega-bar. Recorded emission signal with nanosecond resolution at 850nm wavelength reveals nonlinear increase character. A time resolved absorption coefficient α (t) was taken to fit the radiance curve. The value of α (t) indicates that shock induced shear banding could be regarded as the emission source, seems increasing after the shockwave arriving in the sapphire windows.

Abstract: When aircrafts have hypersonic flights in the atmosphere, high-temperature IR windows become the main factor of complicated aero-thermo-radiation effects, which reduce the performance of IR detection systems, or even make these detection systems fail. Consequently, measuring thermo-radiation characteristics of IR window material is essential for the assessment of aero-thermo-radiation effect. By analyzing thermal radiation transfer in IR windows, a method is proposed to measure thermo-radiation characteristics of IR window materials, and an experimental platform is established. Based on the platform, thermo-radiation characteristics of a sapphire IR window of a MWIR detection system in a hypersonic vehicle are measured. The results indicate that, the thermo-radiation characteristics of the sapphire IR window material in 3.7μm-4.8μm have an approximate cubic relationship with temperature at 100~350. With the rise of temperature, the transmittance of the sapphire material decreases, while the self-radiation increases. Subsequently, the self-radiation can drive detector into saturation easily, of which the influence on the MWIR detection system is bigger than that of the transmittance. And, with thickness increasing, transmittance decreases exponentially, and self-radiation increases, but the increment decreases exponentially. Consequently, as IR windows’ thickness or temperature increases, SNR of the MWIR detection system declines, saturation of IR detectors occurs easily.