Composite Materials Chapter 7. Ceramic Matrix Composites

7. Ceramic Matrix Composites Difference between CMCs and other composites Nonceramic matrix bear a greater proportion of the applied load. Load partitioning depends on the ratio of fiber and matrix elastic moduli, Ef/Em. in nonceramic matrix composites this ratio is very high In CMCs, the ratio is rather low, can be slow as unity.

7. Ceramic Matrix Composites 3. In CMCs, (a) low matrix ductility (b) high fabrication temperature (c) thermal mismatch between components is important

7.1 Fabrication of CMCs Fig 7.1 Schematic of the slurry infiltration process (After L.E.McAllister and W.L.Lachman, in Fabrication of composites, Van Nastrand Reinhold, New York, 1982, p.196

7.1 Fabrication of CMCs

7.1 Fabrication of CMCs Fig 7.2 Effect of hot-pressing temperature on the bend strength of carbon fiber reinforced borosilicate. Fig 7.3 Schematic of melt infiltration process. Fig 7.4 Schematic of chemical vapor infiltration process (after D.P.Stinton and et al, AM.Ceram.Soc.Bull, 65, 305(1986))

7.2 Properties of CMCs Fig. 7.5 Fiber and/or matrix failure : a original situation, b fiber failure, c composite failure controlled by fiber failure, d fiber bridging the matrix cracks

7.2 Properties of CMCs In MMC and PMC : Fiber fail first : Єf(matrix) > Єf(fiber) In CMC : matrix fail first : Єf(matrix) < Єf(fiber)

7.2 Properties of CMCs Table 7.1. : Theoretical matrix cracking stress for a borosilicate glass and magnesia reinforced with 60% of high-modulus carbon fibers. (E=360GPa)

7.2 Properties of CMCs Fig 7.6. Schematic tensile stress-strain curve of an aligned CMC in the longitudinal direction (after A.G.Evans, Mat.Sci.Eng, 71, 3(1985))

7.2 Properties of CMCs 1. Matrix microcracking ∵ fibers have enough strength than matrix 2. Flow stress rises to σu, fiber bundle fails :fiber pull out starts.

7.2 Properties of CMCs In iso-strain condition, ec = ef = em Where subscripts c, f and m denote composite, fiber and matrix. σc = σfvf + σmvm (rule of mixture) Assuming elastic behavior

7.2 Properties of CMCs In CMC, єf(matrix) < єf(fiber) STRESS CARRIED BY FIBER/STRESS CARRIED BY MATRIX =σfVf/ σmVm=EfVf/EmVm In CMC, єf(matrix) < єf(fiber) σo = σfvf + σmu(1- vf) Where σmu is the matrix stress at its breaking point. Rearranging, we have *Disadvantage is that matrix microcracking provides an easy path for environmental of the fiber f/m interface.

7.2 Properties of CMCs Table 7.2 : Properties of Carbon fiber reinforced glass composite. Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag,

7.2 Properties of CMCs Fig 7.7 Relationship between strength and Vf · Linear increase in strength with Vf up to 50 ~ 55%. Beyond 55% Vf the strength deviates from linearity owing to the matrix porosity increased. Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

7.2 Properties of CMCs Fig 7.8 Youngs Modulus as a function of fiber volume fraction continuous, aligned carbon fibers in borosilicate glass matrix Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

7.2 Properties of CMCs -Properties of 50% Sic/LAS After from K.M.Prewo, J.Master.Sci, Eng,17, 3549(1982)

7.2 Properties of CMCs a b (a) Microstructure of hot-pressed Sic/Ba-Si-Al-O-N composite. (b) Firacture surface of SiC/Ba-Si-Al-O-N composite showing fiber pull out : weak fiber/matrix bond. Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

7.2 Properties of CMCs Fig 7.11 Toughness grains obtained by incorporating SiCw in alumina (after T.N.Tiegs and P.F.Bencher, in Tailoring multiphase and ceramics, Plenum press, NewYork, 1986, p.639 )

7.2 Properties of CMCs Fig 7.12 High-temperature strength increase as a function of the Sicw volume fraction in Sicw/Al2O3 (after T.N.Tiegs and P.F.Becher, ibid, 1986) Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

7.2 Properties of CMCs Fig 7.13 Increased Creep resistance of composite(Al2O3/SiCw) and Al2O3 • Al2O3/SiCw shows higher toughness and flexmal strength than monolithic Al2O3. Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

7.3 Interface in CMCs Table 7.3 composite of damage resulting from the thermal expansion mismatch in some carbon reinforced systems. Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

7.4 Toughness of CMCs Fig 7.15 (a) Original state with Frictional gripping of fiber. (b) Crack in the matrix Is momentarily halted by fiber. (c) Interfacial shear and lateral contraction of fiber and matrix result in debonding and crack deflection along the interface. (d) Further debonding, fiber failure at a weak point and further extension (e) Broken fiber ends are pulled out against frictional of interface, leading to total separation. (after B.Harris, Met.sci, 14, 351(1980)) Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

7.4 Toughness of CMCs Table 7.5 Ceramic matrix composites toughning mechanisms. Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)