不同湿度环境下镀钛金刚石烧结聚晶金刚石的摩擦学性能

Tribological Behaviors of Polycrystalline Diamond Sintered by Titanium-coated Diamond Particles under Humid Condition

聚晶金刚石(Polycrystalline diamond,PCD)机具在钻探破岩与切削过程中服役于边界润滑环境,湿度条件是影响其摩擦磨损性能及切削钻进效率的重要因素。采用磁控溅射技术在金刚石微粉表面沉积厚度为~500 nm的钛薄膜,并选用镀钛金刚石微粉为原料烧结聚晶金刚石(Ti-polycrystalline diamond,Ti-PCD)。研究了Ti-PCD在5%~50%相对湿度(Relative humidity,RH)条件下对磨氮化硅的摩擦磨损性能,利用SEM、XRD、AES等表征镀钛金刚石微粉和Ti-PCD的微观组织、表面形貌及相结构。采用光学显微镜、白光三维形貌仪、拉曼光谱仪分析Ti-PCD和氮化硅球的磨损形貌。结果表明,Ti-PCD中金刚石晶粒与粘结剂钴界面处形成碳化钛过渡层。在相对湿度为5%~50%RH条件下,氮化硅磨斑处的碳质转移膜是影响Ti-PCD稳态摩擦因数的主要原因。5%RH干燥环境下,摩擦滑移过程中碳原子重杂化过程形成连续均匀的碳质转移膜,获得超低的稳态摩擦因数0.034。Ti-PCD表面相对较疏水,水分子钝化作用减弱,有助于形成具有减摩作用的碳质转移膜,致使湿度环境下的稳态摩擦因数比传统PCD降低~30%。Ti-PCD磨损在5%~50%RH湿度范围内逐渐减轻。Ti-PCD中的碳化钛相发挥结合桥作用,利用界面效应强化粘结剂钴和金刚石的界面结合,抑制摩擦滑移过程中的金刚石颗粒剥落,提高Ti-PCD的耐磨性。应用金刚石微粉表面涂层技术制备减摩Ti-PCD,从界面结合和补强增韧方面强化金刚石与粘结剂钴的界面状态,对设计制造高效长寿钻探机具有重要的研究意义。

The polycrystalline diamond(PCD),sintered at a high temperature(1 460-1 500 ℃) and pressure(5-5.5 GPa),consists diamond particles and a cobalt binder.PCDs are widely used in geological and petroleum drilling systems,such as thrust bearings and drill bits,owing to their high hardness,toughness,thermal conductivity,and wear resistance.In geological drilling,PCD tools are used in a complex boundary lubrication environment in which the relative humidity(RH) significantly affects the tribological behavior and drilling efficiency.Coating diamond particles with strong carbide-forming elements has been proposed to strengthen the interfacial bonding between the diamond and cobalt binder in PCDs,which influences the tribological behavior.However,the tribological behavior of a PCD sintered by coated diamond particles at different RH levels,and the corresponding mechanisms,have yet to be studied in detail.A titanium(Ti) coating with a thickness of ~500 nm was deposited onto diamond particles via magnetron sputtering.The Ti-coated diamond particles were then sintered at high temperature and pressure to synthesize the Ti-PCD.The tribological properties of the Ti-PCD were studied at 5%-50% RH.The Ti-coated diamond particles and Ti-PCD were analyzed by scanning electron microscopy,X-ray diffraction,Auger electron spectroscopy,impact tests,and tribological tests.The wear morphology of the samples was analyzed by an optical microscope,a Nano Map-D three-dimensional white-light interferometer,and Raman spectroscopy.The results showed the formation of a titanium carbide transition layer between the diamond grain and cobalt binder in the Ti-PCD.The as-obtained titanium carbide phase promoted the tribological behavior of the Ti-PCD under humid conditions,including the reduction of the friction factor and enhancement of the wear resistance.The friction factor of the Ti-PCD tested at different RH levels had a run-in period and steady state.The friction factor significantly fluctuated during the run-in period and stabilized after ~8 min.The steady friction factor exhibited a rising trend with the RH level,increasing from 0.034 at 5% RH to 0.073 at 50% RH.The steady friction factor of the Ti-PCD at 5%-50% RH was affected by the carbonaceous transfer film on the worn silicon nitride surface.A low steady friction factor was generally accompanied by a high transfer film-covering fraction.A continuous transfer film induced by carbon hybridization was formed at 5% RH,leading to the lowest steady friction factor.A carbonaceous transfer film was formed by the layer-shearing action of massive tiny diamond grains exfoliated from the Ti-PCD surface,which significantly reduced the friction factor.The Ti-PCD surface was lyophobic,and the H_(2O) dissociative passivation was weak.This facilitated the formation of the carbonaceous transfer film,which reduced the friction factor by 30% compared with that of pristine PCD(P-PCD) in humid environments.The wear rate of the Ti-PCD decreased from 2.4 × 10~(-11) mm~3/(N·mm) at 5% RH to 4 ×10~(-12) mm~3/(N·mm) at 50% RH.Diamond exfoliation during the sliding operation dominated the wear loss of the PCD,leading to massive spalling pits on the wear track.The Ti-PCD wear was much milder than that of the P-PCD,exhibiting the same turning trend as the RH level.The wear rate of the Ti-PCD at 5% RH was significantly lower than that of P-PCD(9.1 × 10~(-11) mm~3/(N·mm)).The high-temperature and high-pressure sintering of Ti-coated diamond particles significantly enhanced the wear resistance of the Ti-PCD.The titanium carbide phase inhibited diamond exfoliation by strengthening the interface bonding,which involved transforming the mechanical interaction between the diamond grains and cobalt binder into chemical bonding,thereby enhancing the wear resistance of the Ti-PCD.These results indicate that the tribological behavior of the Ti-PCD in humid environments can be significantly improved by introducing Ti-coated diamond particles,which mainly transfer the interfacial state of diamond particles and Ti-PCD.The Ti-coated diamond particles were used to synthesize the anti-friction Ti-PCD.The results showed that the interface state between the diamond grains and cobalt binder was strengthened by interfacial bonding,strength,and toughness reinforcement.This will be significant in the fabrication and application of efficient and durable drilling equipment.