1、Refractories and Industrial Ceramics Vo/3&3,bs.3-4.l O0 UDK 666.762.11:620.17 EFFECT OF THE SIZE OF SiC WHISKERS ON THE OF ALUMINA-BASED COMPOSITE MATERIALS TOUGHNESS S.M.Barinov,1 A.V.Klov,1 V.Ya.Shevchenko J.Dusza,z I.Rudnajova,2 O.N.Grigorev,3 and N.A.Orlovskaya 3 Translated from Ogneupory i Tekh
2、nicheskaya Keramika,No.3,pp.t0-13.March,1997.Originai ar:icle submitted October l O,1996.The effect of The dimensions of SiC whiskers on the strenh and toughness of hot-pressed composites with a 20%volume fraction of whisker reinforcement was studied.It was established that the mechanical properties
3、 of the composites were improved only on introducing whiskers with the diameters within 1.5-2.5 gm.These materials also exhibited a higher resistance to crack propagation.Crystais orsmaller diameter had a negative effect on the mechanical properties,which was explained by bundle tbrmation.There is a
4、 definite criticai content of whiskers ensuring*,heir uniform distribution in the matrix,and this vaIue decreases on reducing the whisker diameter.A correlation was established between composite toughness and the level of average residual compressive stresses in the whiskers.Introduction.Reinforceme
5、nt of ceramic matrices by whiskers leads to increased toughness,strenh,and resis-tance to wear and fatigue fracture and improves some other properties determined by crack nucleation and propagation in the material(see 1,2 and 3,pp.119-138).The effect of whiskers on the mechanical properties of the m
6、atrix material is explained by two principal mechanisms.First,by resis-tance to crack opening at the expense of inelastic drawing of whiskers from the ceramic matrix or elastic delbrmation of the whiskers.Second,by reorientation of a crack upon its in-teraction with the whisker-matrix phase boundary
7、.The question as to which mechanism is dominating is still under discussion,although it is evident that manifestations of both mechanisms must depend on the shape and dimensions of the whiskers,their binding to the matrix material,and the level of residual stresses in the composite.In this connectio
8、n,the purpose of this work was to study the role of the geometric parameters of SiC whiskers on the toughness of whisker-reinforced A1,O 3-SiC composites.Materials and methods,Samples of composites were obtained by hot pressing at a temperature of 1700 and a pressure of 30 MPa in graphite molds char
9、ged with a mixture i lnterbranch Research Center for Commercial Ceramics of he Russian Academy of Sciences,Moscow,Russia.2 Insti,ute of Materials Science,Academy of Sciences of Slovakia,Bratis-lava,Slovakia.Institute of Problems in Materials Science,National Academy of Sciences of Ukraine,Kiev.Ukrai
10、ne.ofcornmerciaI alumina powder and 3-SiC whiskers.The ave-rage geometric parameters of the whiskers are given in Ta-ble 1.Prior to charge preparation,the whiskers were thor-oughly treated in hydrofluoric acid to remove the oxide sur-face layer.The components were mixed in suspensions with a control
11、led pH,followed by filtration and drying in vacuum.The volume fractions of the components in the charge were 80%At_,O 3 and 20%SiC.After hot pressing,the level of residual stresses in the samples was estimated by the x-ray diffraction method.The residual stress was caused by mismatch of the coeffici
12、ents of linear thermal expansion and elastic properties of A120 3 and SiC.The measurements were performed on polished sample surfaces with the aid of a DRON-4 diffractometer using Cu3.radiation.A thin layer of silicon powder deposited on the sample surface served as a reference.The average resid-ual
13、 stresses in AI=O 3 were evaluated assuming isotropic elas-tic properties(Youngs modulus 407 GPa;Poissons coeffi-TABLE 1.Parameters of SiC Whiskers and Densi W of AI203-SiC Composites Whisker diameter,Whisker length,Composite tam tm densitF.g/crn 3 0.1 10 3.74 0.3-0.4 I0-15 3.76 1.5-2.5 l 5-40 3.80
14、AI:O3 densiD 4.00 glcm 3.96!083-87:97/3803-096S I g.O0 C 1997 Plenum Publishing Corporauon Effect of the Size of SiC Whiskers on the Toughness of Alumina-Based Composite Materials 97 cient 0.23),using Hookes law and residual detbrmations de-termined by conventional methods 4.The average residual str
15、esses in SiC whiskers were evaluated proceeding from the condition of equilibrium in heterophase systems.The mechanical properties of the sampies were studied on a UTS-100 testing machine by the mehod of three-point bending with a distance of 20 man bewveen the points of load-ing.The sample dimensio
16、ns were 3 x 5 40 ram_ The tests gave values of the flexural strenh and the toughness:the lat-ter parameter was determined on samples having a side notch of 0.5 mm(a standard sample configuration tbr the SENB method of fracture mechanics)?9 The notch vertex radius was about 50 am.We also studied curves of resistance to crack propagation(Kp,curves)measured by the IS technique.The stress inten-sit?,.coefficient Kis was calculated by the formula?9 l,vR KIs=0.59-tcyP-F.(1)H,where E is the elastic mod
