组装方式对立方氮化硼合成的影响

组装方式不仅可以影响高压腔内温度压力的分布,最终影响合成晶体的质量,而且也直接影响到对原料hBN的选择。本实验用国产六面顶压机,在4～5GPa,1500～1700％范围内研究了组装方式对立方氮化硼（CBN）合成的影响。通过光学显微镜观察了晶体生长的特性及形貌。结果表明,在实验中,用粉末状触媒和六角氮化硼采取分片式组装,CBN的成核会聚集在片与片的交界面处,且生长的CBN晶体多为厚板状八面体。这是由于在粉压成型的过程中,受力不均造成了上下表面的触媒与六角氮化硼的密度高于内部的,且高密度的粉体也有利于温度和压力的传导,所以导致在此处的CBN成核数量高于内部。可见,预压的片越薄,片内各处的粉体密度就越相近。因此,在以粉体为初始原料合成CBN晶体的实验中采用薄片式组装方式,选取hBN粉体堆积密度大的为原料对于提高CBN晶体单位产量更为有利。     

国外立方氮化硼研制技术(Ⅵ)

六、在不同触媒—hBN体系中CBN的合成国外许多研究者对不同的触媒-hBN体系中CBN的合成技术问题做了较为广泛的探讨,本文将对hBN-Ca_3B_2N_4体系中CBN的合成,hBN-Li_3N体系中CBN晶体的成核和生长过程,在含氢体系中CBN单晶的合成,BN-卤化物体系中CBN的合成等问题进行阐述。     

高品级粗颗粒(30/60)CBN的合成技术

采用国产铰链式六面顶压机从HBN原材料、触媒体系及合成工艺等方面对高品级粗颗粒(30/40-50/60)CBN晶体合成技术进行了研究.对HBN原材料进行了高真空度下高温处理,使BN含量达到了99%.对CBN晶体的合成工艺及参数进行了优化设计.生长时间设为1200s～1500s.试验结果表明合成CBN单次产量稳定达到85克拉,30/40-50/60比例达69.20%.光学显微镜观察及扫描电子显微镜(SEM)分析表明,大多数CBN晶体生长发育比较完整,30/40-50/60粒度平均单颗粒静压强度均超过80 N.目前该项技术已成功投入工业生产.     

用Ca3N2和Mg3N2合成cBN

分别以Ca3N2和Mg3N2作触媒进行cBN合成实验,对比了这两种触媒合成cBN的温度压力范围的差异,对得到的cBN晶体的形态、颜色和粒度分布等性质作了检测比较。实验结果表明,在Ca3N2-hBN系统中合成cBN的温度和压力条件较低,cBN稳定生长的温度压力范围也较宽（4．5～5．5GPa,1100～1450℃）,实验的重复率较高,但得到的cBN晶体杂质较多,晶形不好。在Ca3N2-hBN系统中合成cBN所需的温度和压力条件较高,cBN稳定生长的温度压力范围与Ca3N2-hBN系统相比也很窄（5～5．5GPa,1400～1600℃）。Ca3N2作触媒得到的cBN晶体粒度偏细、晶形完整率高,抗压强度高于用Mg3N2合成得到的cBN晶体。用这两种触媒合成cBN时,添加剂（Li、Mg、Ca等具有触媒性质的物质与B、N、H等的化合物）的加入对合成结果有很大的影响,无添加剂存在时,两种触媒合成的试样中,cBN的转化率都很低,晶体的粒度很小;加入添加剂后,随添加剂加入比例的改变,cBN的转化率、粒度和颜色都有很大的变化,添加剂占实验样品重量百分比为0～4％时,cBN晶体转化率逐渐提高,晶体粒度逐渐增大,晶体颜色变深,当添加剂的重量百分比加大至5％～8％时,出现部分深棕色至黑色的cBN晶体,合成实验重复率降低。     

控制自发成核生长大颗粒立方氮化硼晶体

CBN(立方氮化硼)的生长速率很低，用普通方法很难长出大的晶体。在本文中用Mg为触媒，在hBN-Mg体系中加入适当比例的水或酒精控制cBN晶体的自发成核。由于在高温下水能分解六方氮化硼(hBN)，在系统中形成高活性的B和N原子，从而改变了晶体的生长环境，提高了生长速率，使晶体迅速长大，在5．5Gpa和1500℃超高压高温条件下保温8分钟获得了1．6mm的cBN晶体。本文还讨论了在该体系中cBN晶体的生长机制。     

立方氮化硼触媒多样性及生长特性研究

本文详细地介绍了立方氮化硼合成用触媒的多样性,描述了在不同类型触媒,如碱金属,碱土金属及其氮化物,硼氮化物,硼化物,某些盐类乃至氧化物中合成立方氮化硼晶体的特征。实验表明,立方氮化硼晶体的基本形态为正八面体,由于发育不完整而形成正十四面体,正二十二面体,板状八面体,针状晶体以及他们的变形。晶体的颜色主要由添加剂的种类决定,合成条件的变化可改变晶体中杂质含量的多少,从而导致晶体颜色的深浅变化。合成中所采用的添加剂应具如下特点：(1)能有效溶解六角氮化硼的物质,(2)能与六角氮化硼反应生成含氮化硼中间产物的物质。最好二者兼备。     

微波-熔盐热处理在CBN表面形成TiN类石墨烯晶体

用Ti和CBN粉体，通过微波-熔盐处理法在CBN表面反应生成氮化物材料，利用XRD、SEM和EDS对涂层的化学组成、相组成和微观结构进行分析和表征。结果表明:经过熔盐处理，在CBN表面形成TiN、Ti₂N、TiN_0.3和TiB₂组织。粗粒度的CBN表面形成的涂层厚度约2.8 μm，涂层表面有许多微小孔洞；当CBN较细时，CBN表面形成纳米花蕾状组织，花蕾状涂层组织由许多氮化钛纳米棒和片状组织构成。此外，CBN颗粒间还存在许多TiN类石墨烯晶体组织，厚度约40~90 nm。      

NH4F对Li3N-hBN体系中合成CBN晶体的影响

本文研究了在Li3N—hBN体系中加入不同量的NH4F后，对CBN晶体的合成的影响。结果表明，加入NH4F后，合成压力降低了约20％。分析以NH4F和Li3N混合粉末为原料的实验结果后推测：在高温高压条件下NH4F首先会分解为NH3和HF,HF进一步和Li3N反应生成更加稳定的LiF和NH3，其化学反应式可以写成：3NH4F＋Li3N=3LiF＋4NH3，这一过程通过X光衍射分析实验产物得到证实。为了考察LiF在合成立方氮化硼过程中的作用，我们用Li3N和NH4F反应所制得的LiF单独作触媒，在LiF—hBN体系中进行了CBN晶体的合成实验，结果表明在上述压力温度条件下，没有CBN晶体生成，即LiF不宜单独做触媒。尽管在NH4F与Li3N反应消中耗了Li3N—hBN体系中的作为触媒的Li3N，但是，合成压力仍有较大幅度的降低，表明少量氨的存在，使CBN晶体的形成更加容易。     

B-N-Mg体系中CBN的合成

碱金属和碱土金属的氮化物是实现BN转变为CBN的触媒，其中以Li、Mg和Ca的氮化物最为有效。本文将对BN-Mg体系中CBN生长的压力、温度区，B—N—Mg体系中CBN的合成，BN—Mg3N2体系中的反应，以及该体系中CBN形成动力学等问题进行阐述。     

化学反应直接成核生长立方氮化硼

本文用B2O3和单质B作为B源分别与Li3N作用,通过化学反应在低于Li3N-hBN体系合成CBN件下合成出了CBN晶体.在上述反应中最低合成条件分别为4.0 GPa-1400 ℃和4.5 GPa-1450 ℃.由于初始材料中不含BN,所以CBN晶体成核方式为化合物之间或化合物与单质间的化学键重组直接成核而非传统的从sp2杂化的hBN向sp3杂化的CBN相转化过程.实验证明,化学键重组成核所需能量要小于相转化成核所需能量.利用含有硼、氮原子的化合物之间在高温高压下的化学反应直接成核是制备CBN晶体的有效方法.     

Ti_3AlC_2陶瓷结合剂/立方氮化硼复合材料的自蔓延法制备及其组织结构

以Ti、Al、C粉、立方氮化硼磨料为原料,采用自蔓延高温合成法制备Ti3AlC2陶瓷结合剂立方氮化硼复合材料。研究Al的摩尔量、CBN浓度对复合材料制备的影响。通过XRD、SEM、EDS表征方法,对制备的复合材料进行物相及组织结构分析。研究结果表明:添加CBN浓度25%的3Ti/1.2Al/2C的试样,自蔓延反应生成的Ti3AlC2较多,且晶体发育良好。CBN参与Ti-Al-C体系的反应,在CBN表面与基体之间形成了硼化物、氮化物的过渡层,实现了磨料与结合剂的化学键合,提高了基体对磨料的把持力。     

Effects of Edge Preparation and Feed when Hard Turning a Hot Work Die Steel with Polycrystalline Cubic Boron Nitride Tools

Polycrystalline cubic boron nitride (PcBN) is usually employed in hard turning. Selection of optimum edge preparation is machining parameter dependent. This paper investigates the effects of edge preparation and feed on tool life and workpiece residual stresses. An Arbitrary Lagrangian and Eulerian (ALE) finite element model was used to explain tool wear rate and residual stress profile. Experimental results showed that honed edges could be employed for hard turning when tensile principal stresses in the tool were maintained at a low magnitude. Chamfered edges produced less compressive residual stresses on the surface. However, away from the machined surface, compressive residual stresses penetrate deeper into the workpiece.     

An Investigation of Grinding with Electroplated CBN Wheels

Grinding of hardened bearing steel with electroplated CBN wheels was Investigated with particular attention to how the wear of the abrasive grains affects the wheel topography and grinding performance during the wheel ire. Power, surface roughness, and wheel topography data were obtained throughout the wheel life for internal cylindrical grinding. Dulling of CBN gratis by attrition was found to cause an increase in the grinding power, but the degree of dulling was restricted mainly by grain fracture and also by grain pullout. Grain fracture and pullout had a much smaller effect on the progressive increase in active grain density, which caused the surface roughness to progressively decrease. Wheel failure tended to occur by stripping of the abrasive layer when the radial wear reached about 70% - 60% of the grain dimension     

不同PCBN刀具车削硼铸铁的对比研究

使用不同的立方氮化硼刀具材料(PCBN)和刀具结构,在相同加工参数下,对硼铸铁进行切削加工试验,并对比其加工效果。结果表明:同粗粒度的PCBN刀具相比,细粒度的PCBN刀具加工所得的工件表面粗糙度较低,且刀具本身的耐磨性较高;在切削过程中,随着进给量的增大,工件表面粗糙度也增大;PCBN刀具的负倒棱宽度对加工硼铸铁的切削温度和后刀面磨损有较大影响,负倒棱宽度增加,切削温度和刀具磨损增加,而倒棱角度对刀具和切削过程影响有限。     

Development of a patterning system for vitrified CBN wheels based on modal analysis

One of the alternatives under development by industry to reduce tailpipe emissions in internal combustion engines is the texturing of the crankshaft bearings using patterned grinding wheels. As modern industrial grinding solutions for forged crankshafts are based on vitrified CBN wheels, a new approach is needed for rotary dressing patterning. This paper describes the development of patterning system for vitrified CBN wheels based on modal vibration analysis. Aspects related to the device design, modelling and simulation of the texturing process are discussed in the paper. The obtained results reported in this paper indicate a high potential for industrial application.     

Process Requirements for Cost-Effective Precision Grinding

Costs in precision cylindrical grinding are compared for different abrasives, machines and grinding conditions. The analysis is for repeated batch production. Account is taken of machine cost and abrasive cost. Cost comparisons were based on extensive trials to assess re-dress life against workpiece quality requirements. Experiments show that different workpiece materials require different strategies to reduce costs. Easy-to-grind AISI 52100 and difficult-to-grind Inconel 718 materials were ground at conventional speeds and at high speeds. It is shown that wheel speed affects production rate through acceptable values of re-dress life, removal rate and dwell time. Advantages were gained using vitrified CBN at conventional speed and at high speed. For both materials, vitrified CBN wheels used at high speed, gave better quality at lower cost than conventional abrasives. Wheel costs became negligible and labour costs greatly reduced. Re-dress life trials, usually neglected, are shown to be essential to reduce costs and maintain quality [1].     

Evaluation of superabrasive grinding points for the machining of hardened steel

Small diameter grinding points offer greater flexibility for machining free-form contours compared to traditional grinding wheels, despite fewer effective cutting edges. The paper evaluates the influence of grit size (B32, B46, B76), feed rate (125, 250 mm/min) and depth of cut (20, 40 μm) when machining D2 tool steel using electroplated CBN grinding points. Highest G-ratios (～2441) were obtained using B32 tools with corresponding workpiece surface roughness (Ra) of ～0.8 μm after ～6000 mm³ material removed, due to the greater number of effective cutting edges. Attritious wear was the primary wear mechanism although material loading was observed with B76 tools.     

氮化硼纳米管的显微表征及其高分辨成像研究

采用化学气相沉积法制备了多壁氮化硼纳米管（BNNT）,高分辨电镜研究表明,BNNT一个重要的结构特征为层间有序堆垛,且大多以菱面体（r-BN）方式堆垛;多数BNNT具有择优生长取向,管轴沿BN六角网格平面的[1010]方向,即多数BNNT为锯齿型.基于其显微结构特征建立了多壁BNNT的结构模型,并通过计算机模拟其高分辨成像特点,发现r-BN堆垛的BNNT的HRTEM成像效果对其直径、层数以及观察视角等参数敏感,提出也可以通过管身高分辨像出现水纹状条纹直接判别r-BN堆垛.     

Development of a directly-driven thread whirling unit with advanced tool materials for mass-production of implantable medical parts

Medical screws are a common mass-produced implantable medical component made of Titanium. To machine the threads of these types of components, thread whirling with carbide tools is typically used. However, tool wear and low cutting speed limit the productivity and increase the manufacturing cost of such medical parts. In this study, a direct motor driven thread whirling unit for an advanced Swiss-type CNC lathe was developed and it was used with advanced tool materials such as low binder content Cubic boron nitride (CBN) and Polycrystalline diamond (PCD) to find a cost-effective and more productive alternative to carbide tools.     

初始材料中Al含量对CBN复合片烧结行为的影响

探索了以Al作为烧结助剂,压力为5.0 GPa,烧结温度在1 400℃情况下,初始材料中Al的含量对CBN复合片中反应机理和机械性能的影响。通过对CBN基复合片的硬度、微观形貌以及物相的分析,发现CBN和Al的反应、以及CBN复合片的机械性能严重依赖于初始材料中A l的含量。除次之外,本文首次采用晶格常数精确计算来分析了化学反应中B原子去向。实验结果表明:当初始混合粉末中Al的含量不超过15%时,复合片的微观结构都比较均匀,硬度都超过30 GPa。含15%A l的样品具有最好的切削性能。