Co包覆粗晶WC粉体的流化床化学气相沉积可控制备及应用

粗晶硬质合金由于碳化钨（WC）晶粒的粗晶特性，呈现出了比细晶硬质合金更高的韧性、红硬性、抗热冲击以及热疲劳性能，被广泛应用于冲击工具、耐磨耐蚀零部件及硬质合金涂层等领域。然而，无论是硬质合金烧结工具还是耐磨涂层的制备，均存在粗晶特性丧失、组织不均匀、气孔缺陷多等共性问题，严重制约着其服役性能。其根源在于复合粉体的常规球磨制备技术，为获取高的均匀性需要长时间的高能载荷混合，严重破坏了WC的原始晶粒度。如何协同解决WC粗晶粒度维持和两相高度均匀化难以同时兼顾的矛盾是制备高性能硬质合金烧结工具和涂层的关键问题。截止目前，一系列物理改性和化学合成方法被提出，但由于这些技术本身均存在着解决均匀性或者晶粒度的单一性，不能两者兼顾，因此，仍未取得根本性的突破。针对此问题，本研究提出了基于流化床化学气相沉积（Fluidized bed chemical vapor deposition, FBCVD）技术可控制备粗晶Co包覆WC复合粉体的新思路，旨在利用FBCVD不改变颗粒原始晶粒度且能实现两相均匀化的特性，解决现有方法制备WC-Co复合粉体的局限性。取得的主要创新性成果如下：（1）提出并验证了FBCVD用于可控制备高质量粗晶WC-Co复合粉体新思路的可行性。优选了以CoCl2为前驱体的CoCl2-H2-Ar的反应体系，确定了沉积温度范围750~850 °C，CoCl2与H2的进料摩尔比控制在1:5以下。探明了Co的沉积生长机制：Co优先在颗粒的棱角、凸起、台阶以及球磨破碎引入的Co杂质富集处等位置形核沉积，并以岛状生长模式长大。（2）揭示了高温下沉积在WC颗粒表面金属Co的粘结是导致失流出现的根本原因。探明了金属Co的沉积与WC颗粒流化之间的协同竞争关系：WC颗粒的长时间流化有利于Co沉积含量的增加，但Co含量的增加却导致WC颗粒快速失流。发现了温度是协调该竞争关系的主要因素，降低沉积温度虽有利于增加WC颗粒流化时间，但Co的沉积效率较低；提升沉积温度可明显增加金属Co的沉积速率，但会降低WC颗粒流化时间，由此确定了最佳沉积温度为800 °C。实验范围制备得到的复合粉体的Co含量最高为3.44 wt.%。（3）发现了FBCVD在WC颗粒表面沉积的Co催化剂具有强自催化化学镀反应的能力。通过调节FBCVD温度及时间可制备得到含量在0.05~0.72 wt.%，颗粒尺寸在15~50 nm之间的Co催化剂。颗粒大小是影响化学镀Co速率的主要因素，颗粒尺寸越小，催化反应速率越快。确定了FBCVD制备Co催化剂的最佳条件：温度750 °C，时间3 min，其Co含量约为0.09 wt.%，颗粒尺寸约为20 nm。并优化了化学镀反应条件：温度80 °C，pH值为12，络合剂浓度为55.0 g/L，还原剂浓度为100.0 g/L。该条件下的化学镀Co速率高达2.34 mg·g-1·min-1。（4）揭示了新型复合粉体热压烧结过程中的晶粒生长行为，并建立了以维持粗晶特性为主要目的的烧结新制度。Co包覆WC复合粉体大大降低了WC之间的接触概率，有效抑制了固相烧结阶段WC晶粒因相互接触黏结导致的聚集再结晶长大，与球磨复合粉体相比，合金的平均晶粒尺寸下降约8%。优化的热压烧结工艺为：烧结温度1350 °C，烧结压力10~15 MPa。制备的硬质合金具有优异的性能：硬度1267 MPa，断裂韧性14.19 MPa·m1/2，横向断裂强度2383 MPa。（5）证实了新型复合粉体在激光熔覆及等离子体喷涂中具有良好的适应性，并开发了高性能涂层的制备新工艺。新型粉体的Co包覆特性能够有效避免WC颗粒间的相互接触黏结长大、降低高温下WC的直接氧化、减少WC直接与基体接触造成的粉末飞溅，因此能够有效维持粗晶特性、降低涂层孔隙和裂纹的产生、提高涂层的组织均匀性、抑制碳损失以及维持良好界面。与基体相比，激光熔覆涂层硬度提高近5倍，磨损率仅为基体的5%。等离子体喷涂涂层硬度为基体硬度的3.9倍，磨损率仅为基体的10%，大大提高了基体的耐磨性能。;Compared with fine grained cemented carbide, coarse grained cemented carbide has higher toughness and red hardness, higher thermal shock resistance and thermal fatigue properties due to the characteristics of coarse WC grains. Thus they have been widely used in impact tools, wear-resistant and corrosion-resistant parts and cemented carbide coatings. However, there always exist some common industrial application problems such as coarse grain property loss, uneven structure, and pore defects in cemented carbide tools and wear-resistant coatings, which severely restrict their service performance. The root cause of the above-mentioned common problems lies in the conventional ball milling technology process used to prepare composite powder. In order to obtain high uniformity, long-term high-energy load mixing is often required, which seriously damages the original grain size of WC. So, how to synergistically solve the contradiction between the maintenance of WC coarse grain and the high homogeneity of the two phases is a key issue in the preparation of high-performance cemented carbide tools and coatings. Up to now, although a series of physical modification and chemical synthesis methods have been developed, these technologies cannot simultaneously solve the maintenance of WC coarse grain and the high homogenization of the two phases. Therefore, no fundamental breakthrough has been made.In view of the above problems, this thesis proposed a new idea based on FBCVD technology to realize the controllable preparation of high-quality coarse grained Co-coated WC composite powder. The aim was to utilize the characteristics of FBCVD that can realize uniform distribution of the two phases without changing the original WC grain size to solve the limitations of the existing methods for preparing high-quality WC-Co composite powder. The main innovative findings achieved are as follows:(1) The feasibility of FBCVD technology for the controllable preparation of high-quality coarse grained WC-Co composite powder was proposed and verified. The CoCl2-H2-Ar reaction system with CoCl2 as the precursor was optimized. The deposition temperature ranged from 750 to 850 °C, and the molar ratio of CoCl2 and H2 should be controlled below 1:5. The deposition and growth mechanism of Co was proved. It was found that Co preferentially nucleated and deposited at the edges, protrusions, steps of WC particles, and the enrichment of Co impurity introduced by ball milling with a Volmer-Weber growth mode.(2) It was revealed that the adhesion of the metal Co deposited on the surface of the WC particles at high temperature was the fundamental cause of defluidization. The complex relationship between the deposition of Co and the fluidization of WC particles was synergistic and competitive. The details were as follows: the long-term fluidization of WC particles was beneficial to increasing of the deposition content of Co, but the increase of the Co content would cause defluidization quickly. It was found that deposition temperature was the main factor for coordinating this competitive relationship. Lowering the deposition temperature was beneficial to increase the fluidization time of WC particles, but it would decrease the Co deposition efficiency. Increasing the deposition temperature could significantly increase the Co deposition rate, but it would reduce the fluidization time of WC powder. The optimal deposition temperature was determined to be 800 °C. For the conditions tested, the maximum Co content of 3.44 wt.% was obtained.(3) It was found that the Co catalyst deposited on the surface of WC particles by FBCVD had a strong autocatalytic ability in electroless plating. Co catalyst with content of 0.05~0.72 wt.% and particle size of 15~50 nm could be prepared by adjusting the FBCVD temperature and time. Co particle size was the main factor affecting the electroless plating rate. The smaller the particle size, the faster the catalytic reaction rate. The optimal deposition temperature and time for the preparation of Co catalyst by FBCVD was 750 °C and 3 minutes respectively, corresponding to the Co content of about 0.09 wt.% and the particle size of about 20 nm. The optimized parameters of electroless plating conditions were as follows: temperature of 80 °C, pH value of 12, complexing agent concentration of 55.0 g/L and reducing agent concentration of 100.0 g/L. The electroless plating rate of Co under this condition was as high as 2.34 mg·g-1·min-1.(4) The grain growth behavior during the hot pressed sintering process of the new composite powder was revealed, and a new sintering system was established with the main purpose of maintaining the coarse grain characteristics. Co-coated WC composite powder greatly reduced the contact probability of WC particles, which effectively inhibit the aggregation and recrystallization of WC grains caused by mutual contact and bonding during the solid phase sintering stage. Compared with the ball milled composite powder, the average grain size of the alloy was reduced by about 8%. The optimal hot pressured sintering temperature and pressure for the preparation of cemented carbide was 1350 °C and 10~15 MPa, respectively. The cemented carbide prepared under these conditions had excellent mechanical properties of hardness as 1267 MPa, fracture toughness as 14.19 MPa·m1/2 and transverse rupture strength as 2383 MPa.(5) It was proved that the new composite powder had good adaptability in laser cladding and plasma spraying, and a new preparation process of high-performance coating was developed. The metal Co coated on WC particles could effectively avoid the contact among WC particles, reduce the direct oxidation of WC at high temperature and reduce the powder splash caused by the direct contact of the hard phase WC with the substrate. Therefore, it would effectively maintain the coarse grain characteristics, reduce the generation of pores and cracks in the coating, improve the uniformity of the coating structure, inhibit carbon loss and maintain a good interface. The hardness of the laser cladding coating was nearly 5 times higher than that of the substrate, and the wear rate was only 5% of the substrate. The hardness of the plasma spraying coating under this condition was 3.9 times that of the substrate, and the wear rate was only 10% of the substrate, which greatly improved the wear resistance of the substrate.