Mechanical properties of nanostructured Cr-B films produced by RF sputtering

Abstract

Much attention has been focused on nanostructured materials and some novel properties such as inverse Hall-Petch effect have been reported so far, although little information concerning thermal stability at high temperature has been obtained for nanostructured supersaturated solid solutions. The objectives of this paper are to come to a basic understanding of these characteristics in nanostructured Cr-B alloys thin foils. The magnetron RF (Radio Frequency) sputtering method was utilized to produce Cr-B alloy thin foils ranging 10 to 20 micrometer in thickness using a pure Cr (99.9 mass percent) target and pure B (99.9 mass percent) pieces added to it to change composition. The base pressure was below 2.O x 10(exp -5) Pa, and the Ar gas pressure for sputtering was O.8 Pa. Microstructural evaluation was performed utilizing XRD (X Ray Diffraction), EPMA (Electron Micro Probe Analysis) and TEM (Transmission Electron Microscopy) techniques. Grain size was measured by XRD peak analysis using the Scherrer equation as well as TEM observation. Mechanical properties were evaluated with microhardness tests at room temperature. XRD analysis revealed that the alloys showed crystalline states up to 9.O atomic percent B and an amorphous phase appeared beyond this content. The crystalline specimens decreased grain sizes from 40 to 9 nm as B content increased, and a strong (1-1-O) texture appeared regardless of the content. Microhardness of the specimen increased with the solute content up to 5 atomic percent and showed maximum values of 20.5 GPa which decreased for further contents. As grain size also changes with B content, so nanosize effect may have an dominant role in the range of B content from 5.O to 9.O atomic percent in this alloy where the grain size is less than 50 nm.

ナノ構造材料に多くの注目が集められ、逆Hall-Petch効果の様な幾つかの新しい性質がこれまで報告されているが、ナノ構造の過飽和固溶体に対する高温での熱安定性に関する情報はほとんど得られていない。本報の目的はナノ構造Cr-B合金薄箔のこれらの性質について基本的な理解を得る事である。純Cr(99.9質量百分率)のターゲットを使用し、組成を変えるため純B(99.9質量百分率)片をターゲットに加え、10から20マイクロメートルの範囲の厚さのCr-B合金薄箔を作製するためにマグネトロンRF(高周波)スパッタリング法を利用した。基本圧力は20×10(exp-5)Pa以下で、スパッタ中のArガスの圧力は0.8Paであった。微細構造解析の評価はXRD(X線回折)、EPMA(電子プローブX線分析)およびTEM(透過電子顕微鏡)技術を用いて行った。粒子径はTEM観察と共にScherrerの式を使いXRDのピーク解析により測定した。機械的性質は常温で微小硬度試験により評価した。XRDの解析よると9.0原子百分率Bまでは結晶状態を示すが、これ以上のB含有量では非晶質状態が現れる。結晶サンプルではB含有量にかかわらず強い(1-1-0)集合組織が現れた。サンプルの微小硬度は5原子百分率までは溶質原子の含有量と共に増加し、20.5GPaの最大値を示し、更に含有量の増加に対して減少する。粒径もBの含有量と共に変わり、従ってナノサイズ効果はBの含有量が5.0から9.0原子百分率の範囲で支配的な役割を持ち、粒径は50nm以下である。