论文题目：粉末表面涂层陶瓷的硬质合金刀具材料的研制和性能研究
作者简介:
陈元春，男，1973年03月出生，1997年09月师从山东大学艾兴教授，于2000年12月获博士学位。
摘
要
本文从提高硬质合金刀具的耐磨性、探索新型刀具涂层方法入手，突破了在刀具表面进行涂层的传统方法，提出了用溶胶－凝胶法在硬质合金碳化物粉末表面涂覆一薄层氧化铝陶瓷制成复合粉末，然后热压制备刀具材料的新方法，开发成功一类新型的陶瓷涂层硬质合金复合刀具材料。利用多种实验手段深入研究了溶胶的制备工艺和性质，设计了一套有效的粉末涂层以及干燥和分散工艺，并对粉末的涂层机理进行了研究和分析，制定了粉末涂层材料的热压工艺，对材料的微观结构及其对材料机械物理性能的影响进行了较为深入的分析和讨论，并进行了切削实验。
首次研制成功了粉末表面涂层陶瓷的硬质合金刀具材料FTC1、FTC2（TiC基）和FTWC1（(W,Ti)C基），性能尤其是高温性能均比未进行粉末涂层的材料有明显提高。FTC1的平均维式硬度为18.5GPa，平均抗弯强度830MPa平均断裂韧性7.6MPa×m1/2；FTC2平均维式硬度为23GPa，平均抗弯强度800MPa，平均断裂韧性4.98MPa×m1/2，FTWC1平均维式硬度为20.3GPa，平均抗弯强度980MPa，平均断裂韧性5.0MPa×m1/2。
根据胶体化学的有关理论对给定实验条件下异丙醇铝的水解反应机理进行了分析，依此对溶胶制备工艺进行了优化，去离子水[Al(OR)3]:[H2O]=1:60－100，用硝酸作为胶溶剂，用量为[HNO3]:[Al(OR)3]=0.14－0.25 : 1，水浴温度为80℃－90℃，保温48小时。水解和缩聚反应的最终产物为非晶态的一水氢氧化铝（勃姆石），由许多AlOOH相互聚合交联形成的胶核处于中心位置，其外层是紧密吸附的同种离子Al2O22+，它们共同构成了胶粒。胶粒能够吸引溶液中带负电荷的NO3－离子，形成漫散的双电层结构。氧化铝溶胶胶团整体上显示带有正电荷，正是这种胶团之间的静电斥力使胶粒在液相中能稳定地悬浮，共同构成平衡的溶胶体系。
为了制定合理的热处理工艺，对干凝胶试样进行了差热和热失重分析。实验所得干凝胶包含的水分和有机溶剂在100－150℃从凝胶的网络结构中大量逸失；400℃左右有机聚合物开始分解，并逐渐进入结晶化过程；500℃左右转化为g-Al2O3；在1100℃以上开始转变为a-Al2O3，在1150℃保温0.5~1h，可以完全转化为a-Al2O3。
对粉末表面特征、粉末基体与涂层的物理相容性和化学相容性进行了研究，建立了涂层粉末的残余热应力模型，估计了适宜的涂层厚度，在此基础上优化了粉末涂层工艺，并对粉末涂层机理以及Al2O3溶胶在粉末介入条件下的凝胶机理做了较深入的分析，粉末涂层的主要机理是涂层物质中的烷氧基（－OR）和羟基（－OH）会与粉末表面的OH－发生反应，脱去一分子水或一分子异丙醇而生成一桥氧的共价键结构。粉末涂层增厚的机理在涂层初期是紧密层对溶胶中Al(C3H7O)2OH的吸附，在涂层的后期是溶胶胶粒与涂层粉末表面的胶粒发生的胶凝化反应。在TiC晶粒中发现了涂层中氧化铝的扩散层，说明在涂层和粉末的界面除了形成了Ti－O－Al的化学键合外，还发生了物质的扩散，从而形成了致密并且具有一定结合强度的晶态涂层。
对基体粉末的组成和配比进行了优化，并建立了一套相应的涂层和烧结工艺。粉末涂层对提高材料的性能有明显作用，随着涂层厚度的增加，材料的各方面性能均有提高，但当涂层厚度过大时，由于涂层剥落形成尺寸较大且结构疏松的凝胶颗粒，影响了材料烧结过程中的致密化，导致材料性能反而下降。
首次在粉末涂层材料中发现了涂层相的亚晶型组织，亚晶粒的直径从十几纳米到200纳米不等，散布在碳化物晶粒内和晶界上，晶界和亚晶界上没有新相生成。在粉末涂层材料烧结过程中内晶型的形成一方面是由于涂层相强烈的烧结反应引起的收缩导致了涂层界面曲率的明显变化，另一方面是由于碳化物晶界的迁移速率明显高于涂层界面。大量亚晶粒的存在强化了晶界，降低了晶粒自身的强度，因此增加了断裂过程中穿晶断裂的比例，而且在穿晶断裂中裂纹发生偏转和分叉的机会增多，这使得裂纹在扩展时需要消耗更多的能量，另外内晶型结构能显著地阻止碳化物晶粒的长大，细化了材料的组织，减小了临界裂纹的尺寸，这些因素使得材料的强度和断裂韧性均有不同程度的提高。
详细研究了新型刀具的磨损机理和破损机理，粉末涂层硬质合金刀具材料在加工淬硬钢和铸铁时表现出良好的切削性能。粉末涂层使刀具材料切削性能提高的原因一方面是由于材料晶粒的细化、内晶型的形成和尺寸效应的作用，使晶界面积增加，提高了材料抗裂纹扩展的阻力，从而使材料的力学性能和切削性能都有明显的提高；另一方面粉末涂层材料由于均匀分布于碳化物晶粒内和晶粒之间的氧化铝涂层相减小了切屑与刀具之间的摩擦，外露的氧化铝颗粒防止了切屑在前刀面上的粘结，减小了切屑对前刀面的作用力，增加了刀具的耐磨性和高温性能，所以粉末涂层材料在高速切削时其优势更为明显。
关键词：硬质合金，涂层刀具，粉末涂层，溶胶－凝胶法，切削性能
Abstract
In order to improve the wear-resistance performance of cemented carbide tools, a novel cemented carbide tool material was developed by hot pressing with ceramic-coated carbide powders that were prepared from aluminum alkoxide solution and carbide powders by the sol-gel process. In comparison with the conventional method of coating on the tool surface, carbide powders were coated with boehmite sol when dispersed in the mixture of boehmite sol and ethanol. The coated powders were then dried, separated, and hot-pressed. The sol preparation techniques were optimized in experiments and the property of the sol was studied at the same time. A set of technique was designed for powder coating and drying, separating and hot pressing of the coated powders. The coating mechanism was probed. The microstructure of the as received material and its influence on the physical and mechanical properties were discussed and analyzed in depth.
The carbide tool materials hot-pressed by coated powders were developed successfully for the first time which were designated FTC1, FTC2 and FTWC1 respectively. The average hardness, flexural strength and fracture toughness were 18.5GPa, 830MPa and 7.6MPa×m1/2 respectively for FTC1, 23GPa, 800MPa and 4.98MPa×m1/2 respectively for FTC2, 20.3GPa, 980MPa and 5.0MPa×m1/2 respectively for FTWC1.
The mechanism of the hydrolysis of the aluminum isopropoxide [Al (OC3H7)3] was analyzed according to the theory of colloid chemistry, on the basis of which the preparation of the sol was optimized. Al (OC3H7)3 was added to an excess of distilled water (Al : H2O = 1:60-100 ) under vigorous stirring. Nitric acid [HNO3] was then added to peptize the hydroxide precipitate (Al: HNO3 = 1: 0.16). The reaction vessel was then closed and maintained for 48h at 90°C to get clear boehmite sol. The production of the reaction was boehmite (AlOOH). AlOOH polymerized with one another to form the core of the colloidal particle around which the ions in the sol were attached to form the double electric layer. The sol was stably suspended by the electric repulsion between colloidal particles.
In order to establish suitable heat treatment schedule, the dried gel powders were undertaken differential temperature analysis (DTA) and temperature gravity (TG). The results suggested that most of the water and the organic solvent were escaped from the gel network at the temperature from 100℃ to 150℃. The polymer decomposed at about 400℃. The gel began to transform into g-Al2O3 at about 500℃ and further transformed into a-Al2O3 above 1100℃. The gel transformed into a-Al2O3 thoroughly when it was kept at 1150℃ for 0.5-1 h.
The surface characteristics of powders and the physical and chemical compatibility between the powder substrate and the coating were studied. A model of the residual thermal stress in coated powder was established, by which the thickness of the coating was determined. According to the analysis above, the coating technique was optimized. The coating mechanism as well as the gelation mechanism of the sol with the intervention of powders was studied in detail. The mechanism of coating was suggested to be the reaction between the -OH group in the sol and W-OH or Ti-OH on the surface of powder and the formation of [W-O-Al] or [Ti-O-Al] bond. The result of EDAX on the interface between coating and carbide granule suggested that there was diffusion layer in the interface layer. Besides the chemical bonds, the physical diffusion was also one of the coating mechanisms. As a result, the dense coating with relatively high bonding strength was formed finally.
The composition and the proportion of the carbide powder were optimized and the coating and hot-pressing techniques for certain carbide were established. The powder coating could improve the performance of the carbide remarkably. With the increase of the coating thickness, the properties of the carbide were enhanced. While they were weakened when the coating was too thick because the coating tended to shell off under this condition leading to the encumbrance of the densification in heat treatment and the decrease of material properties.
Subcrystals of the coating material with the size from about 10 to 200 nm were found in the carbide crystals for the first time. The subcrystals were dispersed in the carbide crystals and the crystal boundaries. The formation of the subcrystal in sintering was on one hand because the severe contraction of the coating material resulted from the sintering reaction that led to the remarkable variation of the curvature of the interface. On the other hand, it was because the movement rate of the carbide interface was much faster than that of the coating interface. The existence of many subcrystals strengthened the crystal boundaries and weakened the strength of the crystal at the same time, leading to the increase of the trend of intergranular fracture and the possibility of the deviation and bifurcation of the crack. This situation made the crack propagation need more energy. In addition, the powder coating could encumber the growth of the carbide granules, leading to finer microstructure and the decrease of the size of the critical crack. Those factors mentioned above improved the flexural strength and fracture toughness of the material.
The coated-powder tool material showed good cutting performance in continuous and interrupted turning hardened steel and cast iron. Principally, there were two main reasons for the enhancement. On one hand, the coating process made the microstructure of the material finer, which decreased the size of the critical crack and encumbered the propagation of the microcracks. On the other hand, the tiny coating material dispersed among the carbide granules decreased the friction between the chip and the tool face and prevent the chip from adhering on the tool face, which decreased the force of the chip on the tool face and improved the cutting performance of the tool. The tool life of the coated powder material was about two times that of ceramic-coated carbide tool LB01, which presented great potential in machining hard materials.
Keywords: Cemented carbide, Coated tool, Coated powder, Sol-Gel method, Cutting performance
