Ceramic Matrix Composites

Ceramic Matrix Composites (CMCs)

• Exhibit excellent wear and corrosion resistance in various environments and temperatures
• Offer higher strength-to-weight ratio
• Retain higher strength at elevated temperatures
• Possess higher chemical stability
• Fail in a non-catastrophic manner
• Have high hardness and are lightweight

Disadvantages of CMCs

• Processing routes involve high temperatures, limiting their use with high-temperature reinforcements
• Designed to improve toughness of monolithic ceramics, which are brittle
• High processing temperature leads to complex and expensive manufacturing
• Coefficient of thermal expansion differences between matrix and reinforcement cause thermal stresses during cooling

Toughening Mechanisms in CMCs

• Crack impeding: crack arresting mechanism due to fibers' higher fracture toughness than the matrix
• Fiber (or whisker) pull-out: fibers have high transverse fracture toughness, causing failure along the fiber/matrix interface
• Crack deflection: weak fiber/matrix interfaces deflect the crack

Chemical Vapour Infiltration (CVI)

• Widely used to develop CMCs
• Deposits solid materials like carbon, silicon carbide, and boron nitride in a porous structure by vapor decomposition
• Similar to Chemical Vapor Deposition (CVD), but involves deposition within a body

CVI Process

• Prepares a ceramic continuous fiber structure (porous preform) as the reinforcement phase
• Reactant gases or vapors are supplied to the reactor, flowing around and diffusing into the preform
• Decomposition of reactants fills the space between fibers, forming a composite material

CVI Reactor

• Consists of three main parts: feed system, heating chamber, and effluent system
• Can be modified to include forced gas flow and a temperature gradient to avoid restriction of gas flow

Types of CVI Processes

• Isothermal/isobaric CVI process: uniform temperature and pressure, but slow due to low diffusion rate
• Temperature gradient (TG-CVI) process: enhances gas diffusivity by leveraging temperature differences