Modelling constrained shrinking and cracking

工业背景：本建议解决了在制造以下系统时遇到的一般性问题：(1)由劳斯莱斯燃料电池系统有限公司(RRFCS)制造的固体氧化物燃料电池是一个多层陶瓷系统。每一层厚约5-10微米，具有不同的孔隙度和成分。这些层是丝网印刷和顺序烧结的。(2)采用溶胶-凝胶法和后续固化工艺制备了包括氧化铟-锡氧化物光学涂层、二氧化硅基防污涂层和含氟防污涂层在内的TWI防护涂层。这些涂层的厚度通常小于1.5微米。(3)用于微型机电系统的1到50微米厚的压电薄膜通常是通过首先使用静电喷涂、喷墨打印或浸渍涂层沉积微粉，然后再进行烧结来制备的。问题定义：问题是如何避免薄膜在干燥、固化和烧结过程中产生裂纹。高温被用来固结薄膜。随着温度的升高，多孔膜和充满液体的薄膜首先由于液体蒸发而收缩，然后由于烧结或固化而收缩。线条缩水率可高达20%。然而，在多层膜中，由于薄膜与衬底的结合以及与彼此的结合，薄膜不能在薄膜表面的平面内自由收缩。收缩是高度受限的，这会导致应力，从而导致薄膜破裂。研究问题：目前的系统远远没有得到优化。由于涉及的材料和工艺变量太多，使用试错法进行优化几乎是不可能的。迫切需要开发一种计算机模拟能力来模拟受约束的收缩和破裂现象。然而，这种能力尚不存在，主要是因为两个原因：(A)现有的建模技术(有限元方法)需要薄膜材料的粘性。这些粘度在很大程度上取决于材料的微观结构，随着薄膜的收缩，材料的微观结构会发生巨大变化。这些数据太难通过实验获得。(B)预测多裂化的科学为时过早。项目组：在RRFCS和TWI的支持下，这项建议将莱斯特大学、萨里大学和克兰菲尔德大学的三个研究小组以及德国的一个进一步研究小组聚集在一起，以解决这些问题，并开发和验证计算机模拟技术。方法：在最近完成的一个博士项目中，研究人员开发了一种开创性的技术，在不知道粘度的情况下模拟随时间变化的收缩变形。拟议的项目是在这一成功的基础上，进一步发展约束收缩技术，并包括多裂解。处理多裂纹的困难将使用所谓的材料点法来解决。这种方法最初是为塑性变形而开发的，但在我们的初步研究中已成功地扩展到多裂纹问题。计算机模型将围绕三个实验案例进行开发。萨里、克雷菲尔德和维尔茨堡将使用三种不同的实验技术来测量模型中所需的材料数据，并验证模型预测。项目影响：该项目将使优化固体氧化物燃料电池、涂层和压电薄膜的设计、材料选择和工艺参数成为可能。更广泛地说，该项目将对脆性材料的多重破裂模型产生重大影响。这些问题包括陶瓷装甲的弹道冲击、土木结构的导弹或爆炸冲击以及所有玻璃结构的安全问题。

INDUSTRIAL BACKGROUND: This proposal addresses a generic problem experienced in the manufacturing of following systems: (1) The solid oxide fuel cell manufactured by Rolls Royce Fuel Cell Systems Ltd (RRFCS) is a multi-layered ceramic system. Each layer is about 5-10 micrometres thick and has a different porosity and composition. The layers are screen-printed and sintered sequentially. (2) The TWI protective coatings, including optical coatings of indium-tin oxide, silica based protective coatings and anti-soiling coatings with fluorine incorporation, are made through a sol-gel and subsequent curing process. These coatings are typically less than 1.5 micrometres thick. (3) Piezoelectric films, between 1 and 50 micrometres thick, for micro electromechanical systems are often made by first depositing fine powders using electrostatic spraying, inkjet printing or dip coating and subsequently sintering. PROBLEM DEFINITION: The problem is how to avoid cracking of the films during the drying, curing and sintering steps. Elevated temperatures are used to consolidate the films. As temperature increases, the porous and liquid-filled films shrink first due to liquid evaporation and subsequently due to sintering or curing. The line-shrinkage can be as large as 20%. However the films cannot shrink freely in the plane of the film surface because of their bounding with the substrate, and with each other in multilayered films. The shrinking is highly constrained which leads to stresses and hence cracking in the films. RESEARCH ISSUES: The current systems are far from being optimised. It is almost impossible to achieve the optimisation using trial and error experiments because there are too many material and processing variables involved. There is an urgent need to develop a computer modelling capacity for the constrained shrinking and cracking phenomenon. However such a capacity does not yet exist mainly because of two reasons: (a) The existing modelling technique (the finite element method) requires the viscosities of the film material. These viscosities strongly depend on the microstructure of the material which changes dramatically as the film shrinks. These data are too difficult to obtain experimentally. (b) The science of predicting multi-cracking is premature.THE PROJECT TEAM: Supported by RRFCS and TWI, this proposal brings together three research groups at Universities of Leicester, Surrey and Cranfield and a futher research group in Germany to address these issues and to develop and validate a computer modelling technique. METHODOLOGY: In a recently completed PhD project, the investigators developed a ground breaking technique to model time dependent shrinkage deformation without knowing the viscosities. The proposed project is to build on this success and to further develop the technique for constrained shrinking and to include multi-cracking. The difficulty to deal with multi-cracks will be addressed using a so-called materials point method. This method was initially developed for plastic deformation but has been successfully extended to the multi-cracking problem in our pilot studies. The computer models will be developed around three experimental case studies. Three different experimental techniques will be used at Surrey, Crainfield and Wurzburg to measure the material data required in the model and to validate the model predictions. PROJECT IMPACT: This project will make it possible to optimise the design, material selection and processing parameters for solid oxide fuel cells, coatings and piezoelectric films. More generally the project will make a major impact on modelling the multi-cracking of brittle materials. Such problems include ballistic impact of ceramic armours, missile or explosive impact of civil structures and safety concerns of all glass structure