Commercial strain gauges obtain a gauge factor of approximately 2 with a compensated temperature coefficient of resistivity (TCR). Therefore, material development for sputtered thin films with a high gauge factor and negligible TCR was conducted. The object for self compensated sensor materials is the combination of a semiconducting material (negative TCR) with high gauge factor and a metal (positive TCR) leading to a TCR close to zero. With nickel containing diamond-like carbon films (Ni-DLC or a-C:H:Ni) and Ag-ITO compounds zero crossing in TCR and gauge factors higher than 10 were achieved
In many industrial applications strain gauges are commonly used for measurements of applied forces or the loading status of work pieces. While commercial strain gauges using polyimide foils can cause errors due to influence of humidity, directly applied thin film strain gauges avoid this problems. Besides the improvement of the signal due to avoiding glue and polymer substrate, that can creep and swell, also the gauge factor can be further improved by using new sensor materials. Indium tin oxide (ITO) is a very promising material especially for high temperature application. This paper presents investigations of Ag doped ITO films. The sheet resistance of the films increased first with increase in the silver content up to 20 at.‐%. Further increase in silver led to a decrease in the sheet resistance reaching less than 10 Ω for more than 75 at.‐% silver content of the resulting films. The gauge factor showed a maximum of 6.5 at 20 at.‐% Ag content, while the temperature coefficient of resistance (TCR) showed zero crossing for approximately 40 at.‐% Ag content.
Metal containing diamond-like carbon (Me-DLC) is a promising material for temperature compensated thin film strain gauges with high strain sensitivity. The discussed material offers gauge factors above 10 at zero crossing of the temperature coefficient of resistance. With Ni-DLC the best results so far were obtained. Based on the results from static deposition an industrially suited process under dynamic conditions was investigated. In both cases gauge factors of approximately 10 could be realized. But there is still room for further improvement. Additionally, Fe-DLC seems to be an interesting alternative. First investigations show gauge factors up to approximately 8 with reasonable temperature compensation
Heutzutage sind Dünnschicht‐Dehnungsmessstreifen z. B. für den Einsatz in hoch präzisen Drucksensoren weit verbreitet. Spezielle hochempfindliche piezoresistive Nanokomposit‐Schichten, die aus metallischen Nanopartikeln, eingebettet in eine isolierende Matrix aus diamantähnlichem Kohlenstoff (DLC), bestehen, können die Messgenauigkeit deutlich erhöhen. Wesentliche Parameter bei der Charakterisierung der Dünnschichten sind die Dehnungsempfindlichkeit (beschrieben durch den k‐Faktor) und der Temperaturkoeffizient des Widerstands (TCR). Der k‐Faktor von herkömmlich verwendeten NiCr‐Legierungen liegt bei ca. 2. Metall‐DLC‐NanokompositSchichten haben im Labormaßstab bereits eine 5‐ bis 10‐mal höhere Dehnungsempfindlichkeit in Kombination mit einem TCR nahe Null erreicht. Zunächst wurden die hochempfindlichen Schichten mittels statischer Beschichtung in einem Box‐Coater hergestellt, wobei sehr lange Prozesszeiten nötig waren. Durch die Verwendung eines dynamischen Prozesses in dem gleichen Beschichtungssystem konnte der Probendurchsatz leicht erhöht werden. Für den Einsatz dieser hochempfindlichen Schichten im industriellen Maßstab ist allerdings eine wesentlich höhere Effizienz erforderlich, weshalb der Prozess auf eine industrielle Kurztakt‐Hochrate‐Sputter‐Anlage übertragen wurde. Zusammen mit einer Steigerung der Dehnungsempfindlichkeit gegenüber dem dynamischen Prozess im Box‐Coater konnte in dieser neuen Sputter‐Anlage ein mehr als 20‐mal höherer Probendurchsatz erreicht werden.
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