Question

9) (2 marks) Boeing is designing a new passenger jet. The gas turbine jet engine contains turbine blades that must have the following properties. 2000 MPa compressive strength at room temperature 1000 MPa compressive strength at 1000°C Withstand thermal shock of more than 300°C during start up and shut down of the engine. Because the component fits tightly inside the engine housing, the 10 cm long turbine blades must not expand more than 400 microns between room temperature 25°C and the maximum operating temperature 1000°0C The material selection charts on the following pages will be helpful in answering the question.

Answer 1

Answer: Ceramic materials can be used for the turbine blades in gas turbine jet engine.

Reason:

1) Its a known fact that ceramics materials are good in compression than tension. It is given that it should have around 2000 MPa of compressive strength at room temperature. From the first graph, we can see that only ceramic materials and cermet (ceramics+metals) have got strength more than 2000 MPa. If we compare these both, ceramic materials have got less density than cermat and hence it can give less weight and high strength.

2) Ceramic materials have got more resistance towards heat. It is got excellent heat resistance capacity. From graph 2, we can see that ceramics are high thermal shock resistance materials. Also the maximum thermal strain allowed is (10.04-10)/10 = 0.004. i.e the material should have 0.004 expansion per unit length. So from graph 2, we can see that the engineering ceramics have got the least coefficient of thermal expansion which means the expansion is very less.

3) It is given that, the material should posses 1000 MPa strength at 1000 degree celcius. From the third graph, we can see that only engineering ceramics has got this property. It can maintain more than 1000 MPa strength at very elevated temperatures more than 1000 degree celcius.