PREPARATION FOR STRENGTH MEASUREMENTS

4.3. PREPARATION FOR STRENGTH MEASUREMENTS

4.3.1. Nano Powder Preparation and Characteristics

The details of the procedure to prepare nano powders of Acheson type α- and β-silicon carbides with both boron carbide and aluminium nitride as dopants with 1 wt% carbon are given in chapter 2. The method of preparation of nano particles of SiC, purification, mixing with green binders and press- ing in a isostatic press at 250 MPa and then subsequent sintering at 2050 - 2100°C for 15 min under vacuum atmosphere (3 mbar) are all given in chapter 2 in details. The same materials are characterized for their mechanical properties in order to see the effect of introducing nano particles of SiC on these properties of the final sintered material. Here, only the following details of the strength measurements will be given, since these details are important for analysis of the mechanical data concerning nano- materials.

4.3.2. Strength Measurement

4.3.2.1. Flexural Strength

The measurement of flexural strength requires the following steps :

1. Preparation of the Samples,

2. Introducion of 4-Point Bending Samples, and

3. Measurement of the Fracture Load. The measurement of the flexural strength at room and high temperatures was made by four point

bending method using high temperature Bending Strength Tester (Model - 422-S, Netzsch, Germany). It consists of the following main parts :

(a) The Frame, (b) Electrically Heated Split Furnace (up to 1400°C),

MECHANICAL PROPERTIES

157 (c) The Loading Device,

(d) Mesurement System with Inductive Displacement Transducer, (e) The Recording and the Control Console.

The test bars of size of 45 × 4.5 × 3.5 mm 3 were machined with a diamond wheel. Each surface of every sample was ground by silicon carbide powder (– 400 mesh). The bars were finally polished on diamond lapping disc with 1 µm diamond paste to a roughness of 0.01 mm or less. The tensile edges were chamfered (45°) with a similar abrasive.

The 4-point bending tests were carried out on a 40 mm outer span and 20 mm inner span width at a transverse speed of 0.05 mm/min at room temperature and also at high temperatures. On attainment of the temperature, the load was applied at 1.25 N/sec. The measurements were carried out from room temperature to 1200°C in air. The deflection was recorded by the X-Y recorder until the fracture oc- curred. From the fracture load and the sample dimension, the strength was calculated utilizing the fol- lowing equation :

σ = (3P

f . d)/(B w )

(4.16) where, P f = fracture load, d = bending arm = (L 1 –L 2 )/2, L 1 = major span , L 2 = minor span, w =

specimen height, B = specimen width.

4.3.2.2. Fracture Toughness

A large number of methods of measurement of fracture tughness and such other parameters have been standardized for metals and alloys in recent years. These methods are normally suitable for ambi- ent laboratory conditions. Different methods utilize different configurations. In the present work for sintered nano-crystalline particles of silicon carbide, the measurement of fracture toughness was made by 4-point ‘Single Edge Notch Bend’ (SENB) specimen technique.

The load required for causing the fracture of the sample was measured by the above high tem- perature ‘Bending Strength Tester’, having a length 1600 mm x width 750 mm × height 2100 mm. The bars of size ( 45 mm × 4.5 mm × 3.5 mm ) were notched with a diamond blade of 0.2 mm thickness (100 µm diameter), after grinding in a 25 micron diamond wheel.

The notch dimensions were measured with the help of a traveling microscope. As a result of using 0.2 µm thick blade for notching, the notch width varied from 0.31 to 0.39. The value of a/w ≈ 0.25 - 0.40 mm, where a = notch length and w = notch width. The notched samples were put into the furnace. On attainment of the desired temperature, the load was applied on the ‘Bending Strength Tester’ at the

speed of 1.25 N/sec, and the fracture load was noted. The fracture tougness (K 1C ) was calculated by using the following euation as : K IC = (3P f .d)/(Bw 2 )[1.99 – 2.47(a/w)+12.97(a/w) 2 – 23.17(a/w) 3 + 24.8(a/w) 4 ]

(4.17) At least 30 samples were tested at each temperature for a better accuracy of the fracture tough-

ness data in order to give a better interpretation of the fracture mechanical behaviour of the sintered silicon carbides.