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Evaluation of Ceramic Matrix Composite Edge and Surface Damage

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Key Engineering Materials
Authors:
Ralf Goller at Technical University of Applied Sciences Augsburg
Ralf Goller
  • Technical University of Applied Sciences Augsburg
Achim Rösiger at Technical University of Applied Sciences Augsburg
Achim Rösiger
Yäsin Ahmed Mohamed Azzaz at Technical University of Applied Sciences Augsburg
Yäsin Ahmed Mohamed Azzaz
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Abstract

CMCs have been developed in the 80th for the shuttle programs and commercialized as brake discs from 2000 till today. Since 2017 CMCs are used also in components of flight gas turbines. This application requires highest levels of process and material development and performance. Therefore the knowledge of any process on material damage is of highest importance.The presented work will show a method how to evaluate the quality of edges and surfaces of CMCs created by a special diamond machining operation. The objective is to develop a machining process for CMCs in gas turbine applications and a method for quality assessment for future production.
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Evaluation of Ceramic Matrix Composite Edge and Surface Damage
Ralf Goller1,a*, Achim Rösiger2,b, Yäsin Azzaz3,c
123University of Applied Sciences, Augsburg, Germany
aralf.goller@hs-augsburg.de, bachim.roesiger@hs-augsburg.de, cyesin.azzaz2@hs-augsburg.de
Keywords: ceramic matrix composites, machining, damage, grinding, milling, gas turbine
Abstract. CMCs have been developed in the 80th for the shuttle programs and commercialized as
brake discs from 2000 till today. Since 2017 CMCs are used also in components of flight gas
turbines. This application requires highest levels of process and material development and
performance. Therefore the knowledge of any process on material damage is of highest importance.
The presented work will show a method how to evaluate the quality of edges and surfaces of CMCs
created by a special diamond machining operation. The objective is to develop a machining process
for CMCs in gas turbine applications and a method for quality assessment for future production.
Introduction
Major innovations are driven by global warming and the need to reduce CO2 produced by
burning fuel and natural gas. Gas turbines in airplanes especially produce a very high CO2 /NOx
per load, about ten times more than a respective car transport. The target of future gas turbine
developments is to gain more efficiency by using new high temperature stable materials, which
require less cooling. Ceramic matrix composites have been developed in the 80th for the shuttle
programs, SAENGER and HERMES for hot structures but not for engines. Today the first CMC
application in engines was introduced by GE in the GX10 turbine and applied in the 777 aircraft.
The production of those components and further future components require much more cost
efficient production methods. Since CMC applications are still rare on the market the raw materials
and processes are not optimized. Consequently CMC products are still expensive with prices per kg
ranging from 200 € (CMC brake disk) to thousands of € for turbine components. The final
machining of the CMCs can cause 50% and more of the production cost and bears a high level of
risk because the product is almost ready and has the highest value (scrap is very expensive at that
time). The goal of the presented work is to apply different machining processes to various CMC
materials and to develop a concept to quantify its effect on surface and edge quality. In the
verification phase some prototype samples with micro gas turbine design have been produced and
the method has been applied.
Background and State of the Art of CMC Machining
Many different machining methods exist to machine CMCs. The advantages and limits of the
var ious processes are described in [1]. Depending on material type, shape, tolerance, surface
requirements and quantity laser, water, diamond cutting are applied. Further anisotropic physical
properties due to fiber and matrix architecture various problems can be observed during machining.
Complex shapes like turbine vanes can only be machined by diamond grinding using 5 axis
machine tools [2].
Although CMCs show quasi-ductile fracture behavior, its machining shows typical brittle
behavior, as most mo nolithic ceramics. Additional there are constantly changing cutting condit io ns
depending on the heterogeneous and anisotropic material structure.
As known from previous investigations, this leads to an interaction of different severe mechanical
damage mechanisms like edge chipping and fiber frying [3, 4]. While remaining fibers can be
removed in a further machining step, edge chipping can also strongly weaken the structure.
Examples of machining phenomena are shown in the microscopic images in Fig. 1. These edge
damages are evaluated for different CMCs after grinding and milling.
Key Engineering Materials Submitted: 2019-01-31
ISSN: 1662-9795, Vol. 809, pp 161-166 Accepted: 2019-02-13
doi:10.4028/www.scientific.net/KEM.809.161 Online: 2019-06-27
© 2019 Trans Tech Publications Ltd, All Rights Reserved
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
Tech Publications Ltd, www.scientific.net. (Research Gate for subscription journals-16/04/25,14:55:44)
... ▪ Surface topography: refers to the evaluation of surface texture, roughness, waviness, and lay. ▪ Surface defects: typical machining-induced defects during processing of CMCs, which include delamination, edge chipping, fibre fraying, and microcracks [11,21,24]. ▪ Mechanical properties: includes changes in hardness and residual stresses. ...
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