Synthesis of novel superhard materials under ultrahigh pressure

Review Condensed Matter PhysicsSynthesis of novel superhard materials under ultrahigh pressureChao Xu •Duanwei He •Haikuo Wang •Wendan Wang •Mingjun Tang •Pei WangReceived:28January 2014/Accepted:14May 2014ÓScience China Press and Springer-Verlag Berlin Heidelberg 2014Abstract Superhard materials are solids whose Vickers hardness is beyond 40GPa.They have wide applications in industry such as cutting and polishing tools,wear-resistant coatings.Most preparations of superhard materials are conducted under extreme pressure and temperature condi-tions,not only for scientific investigations,but also for the practical applications.In this paper,we would introduce the recent progress on the design and preparations of novel superhard materials,mainly on nanopolycrystalline dia-mond,B–C–N superhard solid solutions,and cubic-Si 3N 4/diamond nanocomposites prepared under ultrahigh pres-sure and high temperature (HPHT),using multi-anvil apparatus based on the hinged-type cubic press.Bulk materials of all these superhard phases have been suc-cessfully synthesized and are systematically tested.We emphasize that ultra-HPHT method plays an important role in the scientific research and industrial production of su-perhard materials.It provides the driving forces for the light elements forming novel superhard phases as well as the way for sintering high-density nanosuperhard materials.Keywords High pressure ÁSuperhard material ÁNanopolycrystalline diamond ÁB–C–N material ÁSi–C–N composites1IntroductionSuperhard materials are solids whose Vickers hardness is beyond 40GPa.Their synthesis is motivated by a need for robust,chemically diverse materials for industrial appli-cations,ranging from cutting and polishing tools to wear-resistant coatings [1–3].As the hardest known material,diamond and related products have been widely used to fulfill many of these needs.However,diamond also has limitations.For example,it is not effective for cutting steel and related materials due to a chemical reaction with iron.Thus,there is a great desire for new materials that can be used in place of diamond.Cubic boron nitride (cBN),currently the second hardest material in industry,can be used to cut ferrous metals.Yet,it does not exist in the nature and must be synthesized under extreme pressure and temperature conditions,making it quite expensive.There-fore,synthesis of novel superhard phases is not only for scientific investigations,but also for the practical applica-tions [1,3–10].Researchers find that superhard materials are commonly composed of boron,carbon,nitrogen,and oxygen.These light elements can form three-dimensional covalent bonds or networks,which can sustain large shear stress [1,3,6,8,11].By enhancing the large intrinsic strength,people could obtain intrinsically superhard materials.Up to now,most of the superhard materials,for example,diamond [9],cBN [10],B 6O [8],B–C–N [3],B–C materials [7],are synthe-sized under high-pressure and high-temperature (HPHT)conditions.Thus,high-pressure method provides an effective way to achieve novel superhard materials.In this paper,we would introduce the recent research progress on some new superhard materials,such as nanopolycrystalline diamond (NPD)[12],B–C–N superhard solid solutions [13],and Si–C–N composites [14],prepared underSPECIAL TOPIC:High Pressure PhysicsC.Xu ÁD.He (&)ÁH.Wang ÁW.Wang ÁM.Tang ÁP.Wang Institute of Atomic and Molecular Physics,Sichuan University,Chengdu 610065,Chinae-mail:duanweihe@Chin.Sci.Bull. DOI 10.1007//scpultrahigh pressure above 18GPa and high temperature up to 2,500°C using two-stage multi-anvil technologies based on the hinged-type cubic press.2Two-stage multi-anvil technologies based on the hinged-type cubic pressThe development of two-stage multi-anvil technologies and the achievement of extreme P –T conditions are the keys to the synthesis of novel superhard materials.In the high-pressure field,various types of two-stage multi-anvil appa-ratus have been developed,for example Walker-type and Kawai-type multi-anvil apparatus [9,10].Though they are widely used in the high-pressure laboratories,the equipment installations as well as the operation costs are very high.More importantly,their particular framework prevents them from being combined with advanced in situ detection tech-nologies.Therefore,there is a trend for researchers to develop new types of two-stage multi-anvil apparatus based on other frameworks.Based on the hinged-type cubic press,which is widely used in the superhard materials factories,we have successfully developed a two-stage multi-anvil appa-ratus as shown in Fig.1[15].Eight tungsten carbide (WC)anvils (18mm 918mm)with different truncation edge length (8,6,4mm)were used to generate pressure,and the semi-sintered magnesia (MgO)was processed into octahe-dral of different length (14,12,10mm)and then used as the pressure medium.These high-pressure assemblies are com-monly represented as 14/8,12/6,and 10/4assembly.Pyro-phyllite closely surrounds the octahedral as the gasket.Metals with high melting points (for example,molybdenum (Mo),tantalum (Ta),and rhenium (Re))are usually pro-cessed into tubes for heating,and zirconium dioxide (ZrO 2)is used as the thermal insulator [12].The pressure in the sample assembly can be calibrated based on the well-known phase transitions of materials like Bi [16],ZnTe [17],and ZnS [18]at room temperature,while temperature in the sample assembly can be measured directly using a B-type or W-Re-type thermocouple,with a precision of about ±10°C.When the needed experimental temperatures were higher than 2,100°C,temperatures could be estimated according to the electric power–temperature relationship,which might result in ±50°C errors.After years of development,the upper pressure limits of 14/8,12/6,and 10/4are about 16,19,and 22GPa,respectively,and the highest temperature can reach 2,500°C.Figure 1shows the whole structures and operation process of the two-stage multi-anvil apparatus based on the hinged-type DS698MN cubic ing this apparatus,we have synthesized some new superhard materials,such as NPD [12],B–C–N superhard solid solu-tions [13],and Si–C–N composites [14].3Synthesis of novel superhard materials 3.1NPDNPD,regarded as a new generation of high-performance superhard material,refers to the diamond products pre-pared through direct converting of graphite intodiamondFig.1(Color online)The structure and operation process of a two-stage multi-anvil apparatus based on the hinged-type DS698MN cubic pressChin.Sci.Bull.under static high pressure without any catalysts[9,19]. This type of product has a high-purity nanopolycrystalline body with strong and direct bonding offine diamond par-ticles in nanometer level.It not only has good perfor-mances that a natural single crystal diamond has,like high density,high hardness,and transparency,but also has better physical and chemical properties than those of single crystal and traditional polycrystalline diamond products. For example,NPD has higher thermal stability,higher fracture toughness,and better wear resistance[20–22].In the meantime,NPD is composed of nanoparticles,so it is isotropic,and can be arbitrarily processed into various shapes[23].Therefore,NPD has a broad application prospects in thefields of industrial applications and sci-entific research[24–26].In order to synthesize NPD samples,high-purity well-crystallized graphite(99.9995%,Alfa Aesar,USA),with crystalline size of5–20l m,was used as the starting material.In addition,some of the graphite was pretreated by high-energy ball-milling for34h in the argon gas atmosphere.In order to remove the absorbed substances such as harmful gases and water,all the powders were also processed in vacuum with the pressure of about 2.5910-3Pa and the temperature of about1,000°C. After these procedures,the powders were packed into the sample assembly immediately and put into the high-pres-sure apparatus[12].Figure2a shows the optical images of the NPD sample we have prepared under HPHT conditions of about 16GPa,2,300°C[12].It is about1.5mm in both diameter and height,and has a dark gray color.A high-quality X-ray diffraction(XRD)data of the as-synthesized sample were collected as can be seen in Fig.2b.By comparing the d-spacing data of the diffraction rings with the database,we found that the main diffraction peaksfit well with cubic diamond’s(111),(220),(311),and(400)diffraction peaks, indicating that the major phase of the as-synthesized sample was cubic diamond.Besides,two weak diffraction peaks were also found to be the(002)and(004)peaks of hexagonal diamond phase.Because of the ultrahigh tem-perature conditions,the package material(tantalum)reac-ted with the carbon source and thus generated a small amount of Ta–C compounds,as shown in the XRD pattern. By further increasing the temperature to about2,500°C, the hexagonal diamond would not appear in the sample, suggesting that the crystalline graphite can be completely converted to cubic diamond at this P–T condition[12].In comparison,we also synthesized NPD samples using the ball-milled graphite as the starting material.The results showed that even at2,100°C,the milled graphite could completely be converted into cubic diamond.Therefore, high-energy ball-milling pretreatment provides an effective way to lower the needed temperature for the NPD preparations under high pressure and can greatly reduce the NPD synthesis costs in the practical applications[12].Figure2c shows the scanning electron microscope(SEM) image of the polished sample that was prepared under P–T conditions of about16GPa,2,300°C using the ball-mil-led graphite as the raw material.We found that the NPD was fully dense,with grain sizes below100nm,which was consistence with the average crystalline sizes estimated according to the Scherer’s formula.We also tested the Vickers hardness of the well-sintered NPD samples through Vickers indentation method.In the hardness tests procedure, loading forces of0.5,1,3,5,and10kg could be chosen,and indentations for each loading force were repeated in three different areas of the polished surface,with the maintaining duration of15s.As is known,the obtained hardness values depend on the loading ually,the larger the loading force is,the lower for the hardness value is.By drawing the hardness-loading curves,scientists found that as the loading force increases,the hardness would decrease,butfinally tended to afixed value.Then,such asymptotic value of hardness versus load curve was regarded as the credible hardness of the material[27].When the load was0.5and 1kg,the indentations were too small to be detected.By further increasing the loading force,the indenter broke[12]. Thus,we believe that our NPD samples are ultrahard. Recently,we have developed a new method to synthesize NPD samples by direct sintering nanodiamond powders under HPHT.Figure2d shows the hardness test results of the NPD samples synthesized by direct sintering method.As shown in thefigure,when the load forces were0.5and1kg, almost no observable indentations were detected.With fur-ther increasing the loads to3kg,a small indentation could be clearly detected,and the hardness was found beyond 100GPa.By further increasing the loading force to5kg, and even to10kg,we got the asymptotic value of about 86GPa.Therefore,direct sintering nanodiamond powder under HPHT is also an effective method to synthesize ul-trahard NPD.3.2B–C–N superhard solid solutionsBoth diamond and cBN have disadvantages in practical applications.For diamond,it would become unstable in the oxygen-enriched environment around900K.Worse still, diamond would react with the ferrous alloys in the high-speed machining process.For cBN,though it possesses better chemical inertness,its hardness is only half of that of diamond.Thus,in order to obtain an ideal superhard material which could possess the advantages of both dia-mond and cBN,scientists were trying to search for some novel superhard phases,namely diamond–cBN alloy or B–C–N solid solutions[1].Theoretical studies suggest that high-pressure dense B–C–N ternary phases are excellentChin.Sci.Bull.alternative materials with ideal thermal stability and chemical inertness.More importantly,they have hardness much higher than cBN.Therefore,these B–C–N ternary phases would not only resist oxidation and graphitization in the extremely harsh working environment,but also keep stable during the hardened-steel cuttings [3,28].Because of technological importance,researchers are trying to prepare superhard B–C–N phases by sintering various starting materials under HPHT conditions [3,13,28–30].In our research [13],mixture of diamond and cBN,as well as mixture of graphite and hBN,was chosen as the raw materials.In the first set of experimental runs,diamond (Alfa,99.99%,0.5–1.5l m)and cBN powder (Alfa,99.99%,80nm–2l m)were first intensively mixed and pretreated in the vacuum.After packaged by the capsule and set into the high-pressure sample assembly,they were compressed to about 18and 20GPa,and heated to 2,000and 2,200K for 5min.We also pretreated the graphite and hBN through high-energy ball-milling process and finally got the amorphous mixture of the carbon and BN.Fig-ure 3a shows the sample prepared at 20GPa and 2,200K from such amorphous mixture.It was clear that the as-synthesized sample was a well-shaped transparent cylinder,with the height of about 1.5mm and diameter of about 1.2mm.The XRD patterns of samples prepared at various P –T conditions were shown in Fig.3b [13].For the sample prepared at the pressure of 18GPa and temperature of 2,000K,the XRD pattern showed visible bifurcations at the (111),(220),and (311)diffraction peaks.Meanwhile,we can see that there is an obviously broadened diffractionpeak in the d -spacing range from 3.8to 4.7A˚.By further increasing the pressure up to about 20GPa and the tem-perature to 2,200K,we got the sample as shown in Fig.3a,and the XRD data of the BC 4N sample show no peak splitting (Fig.3b).It is worth noting that,comparing with the diffraction data of our raw materials,the obvious (200)diffraction peak as well as other two new weak diffraction peaks were detected,indicating a new single crystalline phase was synthesized.This also indicates that the diamond and cBN grains started to react with each other at 18GPa and 2,000K,while at the P –T conditions of 20GPaandFig.2(Color online)a NPD sample prepared at 16GPa,2,300°C,b XRD of NPD prepared at 16GPa and 2,300°C,c SEM image of the NPD prepared at 16GPa and 2,300°C,d Vickers hardness of NPD sample synthesized at 18GPa,2,300°CChin.Sci.Bull.2,200K,the reaction occurred completely.According to the full width at half height of the XRD peaks,we could also find that,when the starting materials were not fully reacted with each other at 18GPa and 2,000K,the width of half height of the diffraction peaks had little change.However,when the reaction fully happened at 20GPa and 2,200K,the XRD peaks of the sample were obviously broadened,indicating the decreasing of the grain size.According to transmission electron microscope (TEM)observations and the Scherer’s formula,we estimated the average crystalline size of the synthesized sample to be about 40nm,which clearly proved that diamond or other carbon could react with BN above the P –T conditions of about 18GPa and 2,000K [13].The well-sintered BC 4N cylinder has a diameter above 1mm and thus was big enough for its hardness test.In our indentation experiments,when the applied loading force was less than 3kg,no observable indentations could be detected,indicating its superhard nature.Further increasing the loading force up to 5kg,a typical Vickers indentation was observed.By measuring the diagonal of the indenta-tions,and computing according to the Vickers hardness formula,we finally got the average hardness of 85±4GPa,very close to the hardness of diamond (75–100GPa).Our experimental hardness also agreed very well with the predicted hardness of 84.3GPa [31].3.3Si–C–N nanocompositesOne of the most famous superhard materials in the theo-retical prediction was carbon nitride (b -C 3N 4),whose hardness was predicted to be as large as that of diamond [32,33].Scientists have been trying to synthesize this potential superhard material,but as far as we know,no clear proof of successful preparation of carbon nitride was obtained.With a similar chemical formula of carbon nitride,cubic spinel Si 3N 4was synthesized by Zerr et al.[34].In the meantime,spinel nitrides,whose Si atoms are substituted by B,C,O,have been intensively studied theoretically [35–37].Though some of these novel superhard phases have been success-fully prepared,most of them are not.As a potential super-hard material with a predicted hardness of about 52GPa,spinel CSi 2N 4has not yet been prepared successfully [38].Thus,our initial purpose was trying to preparing spinel C 3N 4and CSi 2N 4through HPHT method [14].In our experiment [14],h-Si 3N 4(99.9%,1l m,Alfa Aesar),graphite (99.9995%,5–20l m,Alfa Aesar),or diamond powders were selected as the starting materials and were first intensively mixed in a Si 3N 4/C molar ratio of 1:6.Since the high-energy ball-milling process was usually applied in the mechanochemistry,enhancing the homoge-neity as well as the reactivity of the starting materials,we also treated our raw materials through the high-energy ball-milling process within nitrogen atmosphere for 26h.After that,it was treated under HPHT conditions.Figure 4a shows the C–Si–N sample prepared under the P –T conditions of 18GPa/2,000°C,with h-Si 3N 4/diamond mixture as the starting materials [14].From the figure,we can see that the sample has yellow–green color and shaped into a regular cylinder with a height of 2mm and a diameter of 1.2mm.XRD of the sample was tested as shown in Fig.4b.It was clear that the main phases of the sample were cubic Si 3N 4and diamond.There are weak peaks of Al 2O 3due to the Al 2O 3capsule.This indicated that both h-Si 3N 4and graphite were directly transformed into the cubic structure.However,no evidence was found for the formation of the new C–Si–N compound undersuchFig.3(Color online)a BC 4N sample prepared from a ball-milled amorphous mixture of the graphite and hBN at 20GPa and 2,200K,with height of 1.5mm and diameter of 1.2mm,b XRD patterns of the samples are (I)prepared from the mixture of diamond and cBN at 18GPa and 2,000K;(II)synthesized at 20GPa and 2,200K with a ball-milled mixture of the graphite and hBN;(III)prepared from the mixture of diamond and cBN at 20GPa and 2,200K.The top right inset is the XRD pattern for the starting mixture of diamond and cBN [13]Chin.Sci.Bull.P –T condition.We further checked the sample by TEM as shown in Fig.5and found that the crystalline size of c-Si 3N 4/diamond sample is about 20nm [14].Vickers hardness test was performed as well.When the loading force was about 1kg,the measured Vickers hardness was about 41.0GPa,significantly greater than that of the pure c-Si 3N 4(about 31GPa).More importantly,by sintering composites with nanodiamond,we can not only enhance the toughness of c-Si 3N 4samples greatly,but also got a well-sintered bulk c-Si 3N 4/diamond nanocomposite [14].4ConclusionsIn this paper,we introduced the recent progress on the synthesis of novel superhard materials,such as NPD,B–C–N superhard solid solutions,and Si–C–N nanocomposites under ultra-HPHT using multi-anvil apparatus based on the hinged-type cubic press.Bulk materials were successfully synthesized and superhard.We emphasize that ultra-HPHT method plays an important role in the scientific research and industrial production in the field of novel superhard materials.It provides not only the driving forces for the light elements forming 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