The Report :"The Production and Properties of Metal-Carbon Composite Coatings  with Nano-Crystal Structure written High Deposition Rate Magnetron (HDRM) "

 

Introduction

The application of three-coated fuel particles (TCFP) with multi film preserving coating of PyC and SiC, where SiC both secures the mechanical strength of TCFP and prevents the diffusion of disintegration metal products, in particular,  Pd, is the basic concept of high  temperature gas-cooled reactor (HTGCR) (pls, see Fig. 1)[1].

However, under the increase of the power density and efficiency of TCFP service the latter undergoes very high temperature > 1700оС which makes it problematic for SiC mechanical and chemical resistance to preserve. Therefore ZrC is considered as a replacement for SiC.

According to [2] ZrC in TCFP withstands both high temperature and  more chemically aggressive Pd and other disintegration products better than SiC. Thus, more high temperature  resistance of ZrC provides better fuel burning and consequently improving HTGCR  economics.

 

Deposition of MeC coating

There are two basic processes of SiC/MeC coatings for TCFP for application in HTGCR:

(i)                  chemical vacuum deposition (CVD) utilizing thermal disintegrated chemical compositions: chlorides, iodides, bromides [3]; the technology is rather simple but ecologically harmful and, in some cases, explosive, requires high temperature 450-1600оС to obtain required coatings;

(ii)                vacuum physical evaporation or sputtering materials (PVD)[4];  the technology is ecologically sound, non-explosive and does not require high temperature. Among the PVD technologies of depositing chemical compounds, i.a. carbides, the ionic-plasma magnetron sputtering is more effective and technological (pls, see Fig. 2). 

As a result of long intensive experience we succeeded in creating the system of high deposition rate magnetrons (HDRM) with balanced magnetic field and direct cooling of targets.  Such HDRMs  make it possible to have the power density of discharge up to and > 103 W/сm2 and the deposition rate up to tens mkm per hour. HDRMs are universal and used for depositing coatings of metals, alloys and chemical compounds (carbides, nitrides, carbonitrides, etc; pls, see Fig. 3). The basic particularity of HDRM is high power density of discharge and, as a consequence, a high plasma concentration in the magnetic catching zone as well as intensive target erosion. Furthermore, under certain conditions  the sputtering rate of materials in this zone  becomes equal  [5].

The deposition rate equalization for different materials  occurs under the following conditions:

  • The magnetic field under target is symmetric (balanced);

  • The residual induction of magnetic field  under target, B>300 Gs;

  • The power density of discharge in the magnetic catching  zone, P> 20 W/см2;

  • The target erosion zone is of mosaic structure. 

The time for getting deposition rate equalization depends upon the difference in materials deposition rate and sputtering coefficients. In practice, for the majority of materials, when the difference in materials sputtering coefficients is 2-3 times, the time for deposition rate equalization is 10-15 minutes (the sputtering coefficients of С=0,2 and Zr=0,7 under U=600V). Вased on the above the simplest and ecologically sound technology for getting carbides coatings has been proposed. The technology implies the joint sputtering from one target composed of metal and carbon where the target components concentration in atomic %  links with their areas correlation in the erosion zone. 

The scheme and general view of such mosaic target  is given in Fig. 4.  

To get the ZrC coatings the two types of mosaic targets have been made:

  • 50Zr  : 50C at.% - to get under the Zr-C diagram the ZrC0,9-0,95  composition the target matrix is C where  Zr inserts are put into C-matrix;

  • 80Zr : 20C аt% - to get under the Zr-C diagram the ZrC 0,5-0,6 composition the target matrix is Zr  where  C inserts are put into Zr-matrix;

The sputtering of the mosaic targets MeC is carried out in the power density range  41,7 W/см2 – 125,2 W/см2 under the Argon pressure of 3-5х10-1 Pa.  The max ZrC deposition rate is ~ 25-30 mkm/hour with 80 mm distance. The temperature of surface condensing the thin film is 150-200оС. By using the multi cathode system of mosaic targets the ZrC deposition rate could be in the range of 120-240 mkm/hour.

For comparison: the max ZrC deposition rate of the ZrC target (non-mosaic) is ~  3-5 mkm/hour; in the atmosphere of chemically active gas, like propane, the ZrC deposition rate is 5-7 mkm/hour.

 

The composition and structure of  MeC coating.

The roentgenographic investigation of composition and structure of  MeC coating shows that the sputtering of mosaic target leads to the homogeneous  mixing of the atoms of  C and Me with composing carbides, (See tab. 1.); the percentage of carbide phase depends upon the Zr/C correlation:    

(i) 50Zr : 50C – ZrC0,9-0,95≈96,5±4% weight, cubic type lattice, В-1, period – 4,677х10-1 nm, crystallites size - 50-60х10-1 nm, Hv(0,2-1N)= 28-30 GPa.

(ii) 80Zr : 20C – ZrC 0,5-0,6 = 76,5±0,4 % weight + Zr-23,5±0,4 % weight, cubic type lattice, В-1, period – 4,683х10-1 nm, crystalline blocks size - 120-180х10-1nm, Hv(0,2-1N)= 8-10 GPа.

The results of roentgenographic analysis of the phases are confirmed by the investigation of energy spectrums  of photoelectrons.  

Besides, the increase of the C content and the decrease of Me content brings about, together with the MeC-phase, the appearance of a C-phase (diamond like) with cubic type lattice, period – 3,567х10-1 nm, crystallites size ~ 500х10-1 nm and C-phase (graphite) of hexagonal type (2,511 and 8,241)х10-1 nm; blocks size - 500х10-1 nm.

The level of compressive stresses inside the plain carbide phases as well as  metallic and C phases is high approaching  0,2-0,56 GN/m2.

The typical microstructure of surface and coating fracture of ZrC of 12-15 mkm thickness on Cu is displayed on Fig. 5.

The ZrC deposition occurs at the temperature range 150-200 оС forming coating with nanocrystalline  structure which withstand the temperature up to  900-1200оС, Fig 6.

The nanocrystalline  structure of coatings secures very high oxidation-and corrosion resistant  properties. The oxidation rate of nanocrystalline structure coatings inside the pure oxygen current starts fast growing only  from 750 оК.

In addition to high oxidation-and corrosion resistant properties such coatings which are formed by the joint sputtering of graphite and metal from the mosaic targets give a very low coefficient of friction~ 0,06-0,15. 

 

Resume

More than 10 years intensive experience in the application of high rate magnetron deposition systems to sputter a great variety of coatings types, including coatings on nuclear fuel as well as in the investigation of multi component coatings, i. a. different carbides  clearly shows that the HDRM technology cum mosaic targets is a sound alternative to the chemical CVD technology to deposit  multi film coatings on nuclear fuel particles.

The joint sputtering of graphite and metal is of various versions for the technology: for example,  both the sputtering of plain carbon and carbide films of different structure & composition  and mixed - Carbon-Carbide. The technology which is ecologically safe and technologically universal permits to have a very high deposition rate up to a few tens mkm/hour, as a consequence, a high productivity. 

       Sources

  1. Tchernikov A.S., Permiakov L.N., Kurbanov S.D., & others. 

“Nuclear Fuel for HTGCR based on the micro spheres of plutonium oxcide” – Atomic Energy, Vol.88, #1, Junuary 2000, pp. 35-38.

  1. Kazho Minato, Toru Otawa, and Razuhiro Sawa & others– Irradiation  Experiment on ZrC Coated Fuel Particles for High Temperature Gas-Cooled Reactors – Nuclear Technology, vol. 130, June 2000. pp 272-281.

  2. T. Ogawa, K. Ikawa, and  K. Iwamoto “Chemical Vapor Deposition of ZrC Within a Spouted Bed by Bromide Process” I Nuel. Mater, 97, 184 (1990)

  3. Shiriajev S.A.,Atamanov M.V., Mitin V.S. & others “The Production and Properties of Composite Coatings of Metal-Carbon with nanocrystalline  structure” TPJ, 2002г., vol.72, pp. 99-104.

  4. P.S. Kelly and R.D. Arnell “Magnetron sputtering: a review of recent developments and applications, Vacuum 2000, 56, p. 159.

 

 

                                                                       

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
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