열적 특성을 가지는 세라믹스의 고장메커니즘 및 수명 예측

Abstract

열적 특성을 가지는 세라믹스(Electrothermal ceramics)는 전기-열 변환 특성을 가지는 세라믹스를 의미하며 발열, 흡열, 센서 소재의 세 종류로 분류할 수 있다. 본 연구에서는 발열체로서는 PTC 히터와 세라믹히터, 흡열체로서는 열전 소자, 그리고 센서로서는 NTC 써미스터(Thermistor) 등 총 4개 종류의 세라믹스를 선별하여 신뢰성 관점에서 연구를 수행하였다. 구체적으로, Field 고장품을 수거하여 고장 물리(Physics-of-failure)에 근거한 고장 분석(Failure analysis)을 실시하여 고장 메커니즘을 분석하였다. 이에 따른 2-수준 품질기능전개(2-Level quality function deployment)를 전개하여 Field와 가장 유사한 환경에서 고장을 가속화할 수 있는 시험 방법을 설계하였으며 가속 스트레스 수준, 시료수 등을 선정하였다. 가속수명시험을 위한 시험 장비를 설계, 제작 후 가속수명시험을 실시하였다. 가속수명시험 후 발생한 고장을 Field 고장 분석 결과와 비교 분석하였다. 비교 분석 결과 Field 고장 분석 결과와 가속수명시험에 의해 발생한 고장 분석 결과가 일치해 가속수명시험의 설계와 수행이 적정함을 확인하였다. 최종적으로, 4개의 열적 특성을 가지는 소재의 고장 모드, 가속 모드, 수명 분포 곡선, 형상 모수, 그리고 가속 모델을 정량적으로 구할 수 있었다. 세부적인 실험 결과는 다음과 같다. PTC 히터의 고장 모드는 열피로(Thermal fatigue)균열 전파(Crack propagation)이며 형상 모수 3.8인 와이블(Weibull) 분포이다. 열전 소자의 고장 모드는 전단응력에 의해 열전 소재 및 솔더링 부위의 피로파괴이며 가속 모드는 전압 가속이다. 분포 곡선은 형상 모수 3.7인 와이블 분포이며 가속 모델은 Coffin-Manson 모델이다. NTC 써미스터의 고장 모드는 전극 부위의 금속 확산에 의한 합금이 형성되는 열화이며 수명 분포는 형상 모수 3.8인 와이블 분포, 가속 모델은 온도-전압 가속 모델이다. 세라믹히터의 고장 모드는 우발 고장인 방전(Electrical Discharge) 및 열점(Hotspot)이다. 수명 분포는 지수 분포이다. 자체 발열/흡열체인 PTC 히터, 열전 소자는 형상 모수가 3.7~8로 마모성이 컸으며 우발 고장인 세라믹 히터를 제외하고 모두 와이블 분포를 따랐다. 이론적 고찰에 의하여 열응력은 기계적 응력으로 변환할 수 있음을 증명하여 열적 특성을 가지는 세라믹스의 가속 모델로서 마모 고장일 경우 Coffin-Manson 모델을 사용할 수 있음을 확인하였다. 이종 접합(Heterojunction)의 경우에는 경계면의 고장이 가장 많이 발생한다. 이 경우 이종접합의 설계에서 반영해야 할 사항이 열팽창계수 차이가 아니라“열팽창계수 × 체적탄성계수”임을 확인하였다. 연구 결과 전기-열 변환 특성을 가지는 세라믹스 주요 고장메커니즘을 규명하였고 이에 대한 대책을 수립하였으며 수명을 예측하였다. 이러한 연구 결과를 기반으로 새로운 형태의 열적 특성을 가지는 세라믹스 개발에 적용할 경우 신뢰성 향상에 기여할 것으로 기대된다.|Electrothermal ceramics convert electrical properties to thermal properties and vice versa. They can be used for heat emission, heat absorption, and as sensors. As samples for experiments, a PTC heater and a ceramic heater were used for heat emission, a thermoelectric device was used for heat absorption, and an NTC thermistor was used as a sensor. In the experiments, failure analysis was performed on failed samples gathered from a field, and their failure mechanisms were determined. Based on these mechanisms, two-level quality function deployments were developed, and a life test method, which can reproduce the field failure mechanisms, was designed. In addition, the acceleration and the number of samples were determined. Test equipment was designed and fabricated for an accelerated life test (ALT), and the test was performed. The failure analysis of the failed samples from the ALT was performed and compared with the field failure mechanisms. Based on the comparative failure analysis, it was determined that the failure mechanisms in the ALT were the same as those in the field. This proves that the ALT was designed and performed correctly. The failure modes, acceleration modes, life distributions, shape parameters, and acceleration models were obtained for four electrothermal ceramics. The detailed experimental results are as follows: The failure modes of the PTC heater are thermal fatigue and crack propagation, and the life distribution is the Weibull distribution, with a shape parameter of 3.8. The failure mode of the thermoelectric module is the fatigue of the thermoelectric material (Bi2Te3) and soldering area. The acceleration model of this module is the Coffin– Manson model. The failure mode of the NTC thermistor is the degradation caused by intermetallic compound formed from the diffusion of metallic components around the electrode. The life distribution is the Weibull distribution, with a shape parameter of 3.8, and the acceleration model is the temperature– voltage model. The failure modes of the ceramic heater are electrical discharge and formation of hotspots, which are overstress failure mechanisms. The life distribution is the exponential distribution. The electrothermal materials that have self-heat emission/absorption properties, such as the PTC heater and the thermoelectric module, have high shape parameters ranging from 3.7— 3.8, which are the tendency of wear-out failure. In theoretical studies, it was proved that the thermal stress in an arbitrary shape and volume can be converted into mechanical stress, so that the Coffin— Manson model, which is typically used for fatigue, can be used for thermal fatigue. Failures occur frequently at the boundary of a heterojunction because of the coefficient of temperature expansion (C.T.E) mismatch. In this case, it was identified that the design factor for the heterojunction is C.T.E mismatch × bulk modulus, and not the C.T.E mismatch. In conclusion, the main failure mechanisms of various electrothermal ceramics were determined, and acceleration models and test methods were proposed for them. These results are expected to improve the reliability of newly developed electrothermal ceramics.; Electrothermal ceramics convert electrical properties to thermal properties and vice versa. They can be used for heat emission, heat absorption, and as sensors. As samples for experiments, a PTC heater and a ceramic heater were used for heat emission, a thermoelectric device was used for heat absorption, and an NTC thermistor was used as a sensor. In the experiments, failure analysis was performed on failed samples gathered from a field, and their failure mechanisms were determined. Based on these mechanisms, two-level quality function deployments were developed, and a life test method, which can reproduce the field failure mechanisms, was designed. In addition, the acceleration and the number of samples were determined. Test equipment was designed and fabricated for an accelerated life test (ALT), and the test was performed. The failure analysis of the failed samples from the ALT was performed and compared with the field failure mechanisms. Based on the comparative failure analysis, it was determined that the failure mechanisms in the ALT were the same as those in the field. This proves that the ALT was designed and performed correctly. The failure modes, acceleration modes, life distributions, shape parameters, and acceleration models were obtained for four electrothermal ceramics. The detailed experimental results are as follows: The failure modes of the PTC heater are thermal fatigue and crack propagation, and the life distribution is the Weibull distribution, with a shape parameter of 3.8. The failure mode of the thermoelectric module is the fatigue of the thermoelectric material (Bi2Te3) and soldering area. The acceleration model of this module is the Coffin&#8211