Surface hardening of titanium by thermal oxidation
Surface hardening of titanium by thermal oxidation
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钛的热氧化表面硬化
DOI：
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发表时间：
2004
期刊：
影响因子：
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通讯作者：
X. Wang
中科院分区：
文献类型：
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作者：
W. Yan;X. Wang
钛及钛合金具有优良的耐腐蚀性和生物相容性，在工业和医疗领域得到了广泛的应用。然而，耐磨性差是这些合金的主要问题之一，限制了它们的广泛应用。到目前为止，氮化一直是钛合金表面硬化的主要方法[1-4]。氧也可以通过固溶体强化钛，但缺乏钛的氧化强化的报道。本文系统研究了钛在700 ~ 900 ℃热氧化1-4 h后的氧扩散层，以探索氧扩散硬化工艺作为钛表面改性技术的可行性。从TA 2级工业纯钛板（中国西北有色金属研究所）上切下尺寸为10 mm × 10 mm × 2 mm的试样。所有样品在800 ℃下退火1小时，然后在HF水溶液中酸洗以去除表面氧化物层和下面的氧扩散层。热氧化在空气气氛中在700-900 ℃下进行1-4小时。氧扩散层的表面硬度使用HV-1000维氏显微硬度计（中国上海材料试验机公司）以50-1000 g压头载荷测量。使用50 g压头载荷测量横截面硬度分布。使用PHILIPS X射线衍射仪以4 μ min-1的扫描速度用Cu-Kα辐射分析晶体结构。使用LEICA MPS 60光学显微镜和HITACHI S-570扫描电子显微镜（SEM）观察扩散层的微观结构。所有样品上的表面氧化物被鉴定为金红石TiO 2，如图1所示的700 ℃/1小时处理样品上的氧化物膜的X射线衍射（XRD）图案所示。通过在氧化后在水中淬火，可以通过用SiC纸轻轻研磨来去除所有试样上的氧化膜，暴露下面的氧扩散层。图1示出了具有各种处理的试样的氧扩散层的XRD图案。可以看出，随着处理温度或时间的增加，对应于α-Ti的所有衍射峰都向低角度移动，表明晶格参数增加。详细分析表明，代表c轴的（002）峰移动最多，而代表a轴的（100）峰移动最少。这可能是由于氧原子在α-Ti八面体间隙中的溶解主要引起c轴的伸长而几乎不影响a轴[5，6]。表I列出了根据（002）和（101）衍射峰的2θ值计算的晶格参数以及相关氧浓度的估计值[6]。处理温度越高，时间越长，氧浓度越高。然而，在本研究所涉及的处理条件下，氧浓度没有达到34原子%的最高值，如参考文献[6]所述。图2中的显微照片显示了800 ℃/1小时和900 ℃/1小时处理后试样的氧扩散层。可以看出，在靠近表面的区域中，存在穿过晶界的黑线（由箭头指示）。图2中的硬度压痕显示出明显的硬化效果，甚至在黑线的内侧，这表明黑线不是扩散层和金属基底之间的边界。在
Titanium and titanium alloys have been widely used in industrial and medical fields due to their excellent corrosion resistance and biocompatibility. Poor wear resistance, however, is one of the major problems of these alloys and restricts them from wider application. Nitriding has been the major surface hardening method used for titanium alloys so far [1–4]. Oxygen can also strengthen titanium by solid solution, but reports of the oxidation strengthening of titanium are lacking. In the present study, the oxygen diffusion layers of titanium, which were obtained after thermal oxidation at 700– 900 ◦C for 1–4 hr, were systemically investigated in order to explore the feasibility of an oxygen diffusion hardening process as an alternative surface modification technique for titanium. Specimens of size 10 mm × 10 mm × 2 mm were cut from commercially pure titanium sheet of TA2 grade (Northwest Nonferrous Metal Institute, China). All the specimens were annealed at 800 ◦C for 1 hr, and then pickled in HF aqueous solution to remove the surface oxide layers and the underlying oxygen diffusion layers. The thermal oxidation was carried out at 700–900 ◦C for 1–4 hr in an air atmosphere. The surface hardness of the oxygen diffusion layers was measured using an HV-1000 Vickers microhardness tester (Shanghai Materials Tester Machine Company, China) with 50–1000 g indenter loads. The cross-sectional hardness profile was measured using a 50 g indenter load. The crystal structure was analyzed using a PHILIPS X-ray diffractometer with Cu-Kα radiation at a scan speed of 4 ◦ min−1. The microstructure of the diffusion layers was observed using a LEICA MPS60 optical microscope and a HITACHI S-570 scanning electron microscope (SEM). The surface oxide on all the specimens was identified as rutile TiO2, as indicated by the X-ray diffraction (XRD) pattern of the oxide film on the 700 ◦C/1 hr treated specimen shown in Fig. 1. By quenching in water following oxidation, the oxide films on all of the specimens could be removed by gently grinding with SiC paper, exposing the underlying oxygen diffusion layers. Fig. 1 shows the XRD patterns of the oxygen diffusion layers of the specimens with various treatments. It can be seen that with increasing treatment temperature or period of time, all the diffraction peaks, corresponding to α-Ti, move to lower angles, suggesting increased lattice parameters. Detailed analyses reveal that the (002) peak, representing the c axis, shifts the most, while the (100) peak, representing the a axis, moves the least. This can be attributed to the fact that the solution of oxygen atoms in octahedral interstices of α-Ti causes mainly the elongation of the c axis and hardly affects the a axis [5, 6]. Table I lists the lattice parameters that were calculated from the 2θ values of (002) and (101) diffraction peaks and the estimates of the associated oxygen concentrations [6]. The higher the treatment temperature and the longer the period of time, the higher the oxygen concentration. However, under the treatment conditions involved in the present research, the oxygen concentration did not reach the highest value of 34 at.%, as reported in Reference [6]. The micrographs in Fig. 2 show the oxygen diffusion layers of the specimens after 800 ◦C/1 hr and 900 ◦C/1 hr treatments. It can be seen that in the region near to the surface there is a black line (indicated by an arrow) that runs across the grain boundaries. The hardness indenter marks in Fig. 2 indicate an obvious hardening effect, even on the interior side of the black line, suggesting that the black line is not the boundary between the diffusion layer and the metal substrate. In