Cryogenic Properties Analysis of Abradable Seal Coating

Ke-ping Chen

（1.Institute of Refrigeration and Cryogenics,Zhejiang University,2.Hangzhou Hang Yang Expander Co.,Ltd.)

Abstract：Abradable seal coating 320NS is tested in the liquid nitrogen environment,its hot and cold impact,bond strength,hardness,coefficient of linear expansion and surface roughness meet the technical requirements of abradable sealed coatings in the cryogenic engineering is described.

Keywords：Abradable Seal Coating;Cryogenic;Property Test

0 Introduction

Abradable seal coatings are generally composite materials,which include a base phase and an additive phase.The basic phase is generally a metallic phase,but it can also be a non-metallic phase.Additive phase usually has a self-lubricating effect.Commonly soft phases are polyesters,graphite,diatomite,bentonite,h-BN,etc[1,2].Abradable seal coating are widely used in high temperature field.Such as gas turbine,steam turbine,aircraft engine and so on.It has lots of advantages,for example low costs and easy to maintain.The application of cryogenic field is rarely reported.This paper aims to describe the cryogenic properties which are tested in the liquid nitrogen environment and introduce the application prospect in cryogenic rotating machinery.

1 Material and Methods

1.1 Test Materials

Coating material selection is important for our test.In Fig.1[3],it shows which area can be used for cryogenic field.We choose the 2#AlSi-polymers,ASTM B26 AL 7050 substrate material,and Metco 320NS coating material for a turbine.The typical chemical composition of 320NS is shown in Tab.1.

Fig.1 Operation temperature and materials for different turbomachineries

Tab.1 Test material chemical composition

1.2 Test Equipment and Methods

To spray the coating,we used the Atmospheric plasma spraying technology,the processing is shown in Fig.2[4].

Fig.2 Schematic of atmospheric plasma spraying process[4]

The plasma spray process is basically the spraying of molten or heat softened material onto a surface to provide a coating.Material in the form of powder is injected into a very high temperature plasma flame,where it is rapidly heated and accelerated to a high velocity.The hot material impacts on the substrate surface and rapidly cools forming a coating.This plasma spray process carried out correctly is called a "cold process" as the substrate can be kept low temperature during processing of avoiding damage,metallurgical changes and distortion of substrate material.

2 Results and Discussion

2.1 Bond Strength Test

Tensile bond strength is according to national standard GB/T 8642 Thermal spraying-Determination of tensile adhesive strength.From the Fig.3 and Fig.4,we get the values are 25.4 MPa and 11.9 MPa for each sample,which are tested in the normal temperature and cryogenic environment.

Fig.3 Bond strength test bench

Fig.4 Curve of bond strength test

2.2 Surface Profile Test

The Fig.5 shows the scanning electron microscope 200×image of the test material 320NS,surface roughness is 24.3 μm.

2.3 Coefficient of Linear Expansion

Expansion coefficient test is according to national standard GB/T 5594.3 Test methods for properities of structure ceramicused in electronic components and device based on experimental conditions,coefficient of linear expansion test temperature in range of 25℃～400℃.And the value is about 20.3×10-6～24×10-6,it is very small change.It can be seen from Fig.6.

Fig.6 Expansion coefficient test sample

2.3 Hardness Test

The micro-hardness of the sample is shown in the Fig.7,according to geometrical product specifications (GPS) test standard GB/T 1031 Surface roughness parameters and their values.The average hardness for the original material is 105 HV to 77HV after cryogenic treatment.For the coating,the micro hardness is from 64HV to 46HV.

Fig.7 Hardness results

2.4 Micro-feature Inspect

From the Fig.8,we get the coating immersed with liquid nitrogen,without cracks or debris in the cross section between substrate and coating interface.And the friction and wear test is according to ISO 20808 Fine ceramics(advanced technical ceramics).

Fig.8 Micro-feature of abrasion debris

3 Conclusion

Cryogenic test were carried out for seal coating near the 77K,and the comparison results was as Tab.2.The key parameters or properties were met the target value,it means the abradable seal coating could be used in the cryogenic field in the future.

Tab.2 Test results comparison

1)Cryogenic property of the abradable coating(320NS)is small change comparing with traditional application.

2) Wear debris are very small,it can be easily carried away by cryogenic machine’s air or liquid.

3) Abradable sealing coating can be used in cryogenic rotating machines.