Surface treatments of aluminium joints, adhesive thickness and radiation effects

Transcription

1 Surface treatments of aluminium joints, adhesive thickness and radiation effects F. S. Guarino^, J. M. Kenny^ ^ Materials' Engineering Centre, Perugia University, Term, Italy kenny@unipg.it CERN: European Centre for Nuclear Research, CH-1211 Geneva, Switzerland jrancesco.guarino@cern. ch Abstract Aluminium joints have been used in order to investigate the effects of radiation on curing at room temperature structural adhesives. For many applications, like space satellites, high energy physics experiments, and nuclear power plants, radiation can in fact be so high that the performances of many polymers can be completely compromised. The effects of different surface treatments, together with the effects of adhesive thickness, has been analysed in order to correctly design the tests. Mechanical tests on the so designed joints have been performed up to a ionising radiation dose of 4 MGy. The correlation between fracture mechanics and polymer degradation are discussed. Surface treatments can strongly influence the radiation behaviours of adhesive joints. 1 Introduction The aim of high-energy physics is to probe the structure of the matter. In order to do that, it is necessary to use powerful particle accelerators, which are used to accelerate and therefore increase the energy of different particles and to drive them to collide at so-called collision points. In order to analyse the effects of such collisions and the nature of the particles generated by them, large detectors are normally built around these collision points.

2 72 Surface Treatment GERM [1] is presently designing a new particle accelerator: LHC (Large Hadron Collider). It will be the largest and the most powerful particle accelerator in the world, able to probe the structure of the matter down to the level of 10" meters. To obtain such a resolution, detectors have to work in presence of a high radiation level, especially for the parts closer to the collision point: as illustrated in figure 1, at the end of the ten years of planned life-span of the experiments, the integrated dose will reach IMGy at 7.5 cm from the collision point. j,\j\jk.^\jf I 1.OOE+06 g (0 & 1 OOE+05 o r- C 1 OOE+04 'i : '- ' -? i i!"rh». ; ; : :. : : - * : ; : «)?! : : ' ' -~g. ;, : - i!!.'!' g co 1 OOE+03 %w «T3 g 1,OOE+02 1 OOE+01 i finer 4-nn i i ; ' ^ : : : : : : : Distance (cm) Figure 1: Radiation dose in the ten years of planned life-span of the experiments as a function of the distance from the collision point This radiation dose can sensitively affect the properties of polymers present in the detectors' structure. In particular, the effect of radiation on room temperature adhesives needs to be qualified. In order to do that, joint's shear tests and adhesive's tensile tests have been performed on Araldit AW 106/HV 953 U curing at room temperature epoxy adhesive. Samples have been exposed to different gamma radiation levels between 0 and 4MGy. As one can see, the radiation doses used for the tests were higher than expected in LHC experiments. This was because the test dose rate was higher than in LHC, which is a less dangerous condition: under radiation-oxidation conditions, time/temperature phenomena, including dose-rate effects and post-irradiation effects, can be just as important as molecular structural differences in determining the radiation resistance of many polymers. Another parameter important in determining the radiation resistance of an adhesive joint, is the adherend surface: as illustrated in figure 2, a joint is composed by the adherend, the adhesive, and the interface between the two, which is strongly influenced by the adherend surface.

3 Surface Treatment 73 <*i#ur* in iiywr oy j Figure 2: Adhesive joint parts and correlated failure types If the joint is failing at the interface it is its radiation resistance that one measures during the test, and vice versa. Because the interface is strongly dependent on the surface treatment of the adherend, the joint's radiation resistance can be determined by the surface treatment itself. For this reason, surface treatment, as well as adhesive thickness effects on joint performances, have been previously analysed. 2 Materials and Procedure The utilised ionising radiation source used was "CALLIOPE", a ^Co source characterised by a maximum radiation rate of 27kGy/h (for the nominal installed activity of ** Bq), situated in ENEA Casaccia Laboratories (Italy). Normal room conditions were used for the irradiation; the radiation rate was 4kGy/h. Araldit AW 106/HV 953 U curing at room temperature epoxy adhesive, produced by Ciba Geigy, was tested. Epoxies are in fact the best candidates for high energy physics experiments [2]. Joint shear test and resin's dumbbell test were realised. These mechanical tests were performed on an UTS Testsisteme dynamometer, which used the controlling and measuring system Logitel V Joint's shear tests Symmetric single lap geometry equivalent to the single lap geometry defined in ASTM D standard, was selected. The sample's geometry is illustrated in figure 3-a. Aluminium was used for the realisation of the samples. No pressure was applied during the polymerisation, but thickness control was utilised in order to obtain microns thick adhesive layer. In order to choice the surface treatment to utilise, tests on the effects of different surface treatments on the mechanical properties of the adhesive joint, has been performed. The influence of the adhesive thickness on the mechanical properties of the joints has been also analysed.

4 74 Surface Treatment 2.2 Adhesive Dumbbell test Dumbbell test R 527 (geometry 1) was chosen in order to analyze mechanical properties and radiation resistance of bulk adhesives. The sample's geometry is illustrated in figure 3-b. Special attention was taken during the realization of these 3mm thick samples in order to avoid bubbles. S " %< \ ' jy. \ ^\VVV *\\\ -* ',"V /,\." >.V / r Gr1p area 190mm 12mm f ^ 3 mm i k 63 5 mrn 1.5mm -JP s?js < ' ; \ 3mm 115mm, 150mm, 20 mm \80mm Figure 3: Samples used for mechanical tests 3. Surface Treatment Influence Sand blasting, four different pickles (the compositions are reported in table 1), and a particular anodisation (chromate-aluminum surface layer ) where tested. Results, reported in figure 4, showed a great effect of the surface treatment on the mechanical properties of the joints: pickle 1 corresponds to the highest shear strength, while anodisation corresponds to the lowest one. In the case of anodisation, the fracture interests the boundary between chromatealuminum layer and aluminum (adherend breakage). In all the other cases the failure is in the adhesive-adherend interface.

5 Surface Treatment 75 Sand blasting and pickle 1 have been selected as the surface treatments to use for the irradiation tests. Degreasing Acid solution composition Pickling time Temperature Last washing Pickle 1 MGL ALU: 30mm Caustic soda: 42g/l Gluconate of sodium: 14.4g/l 1 min 60 C De-mineralized water and drying in oven at 85 C Pickle 2 MGL ALU: 30 min Nitric acid: 400ml/l Fluoridric acid: 8.5ml/l 3 min Room temp. De-mineralized water and drying in oven at 85 C Pickle 3 MGL ALU: 30 min Sulphuric acid: <H1.82:15% Bicromate of sodium: 7.5%bw 20 min 50 C De-mineralized water and drying in oven at 85 C Pickle 4 MGL ALU: 30 min Chromic acid: 80g/l Sulphuric acid: 3ml/l Pickle 1:30 sec Pickle 2: 2min Pickle 4: 30 sec. De-mineralized water and drying in oven at 85 C Table 1: Pickle's composition #" </> Anodisation S, Blast. Pickle 1 Pickle 2 Pickle 3 Pickle 4 Degreasing Surface treatment Figure 4: Effects of the adherend surface treatment on the mechanical properties of the adhesive joints 4 Adhesive Thickness Influence Shear strength has been measured as a function of the adhesive's thickness. Sand blasting has been chosen as the surface treatment for these tests. Results are showed in figure 6. As one can see, the thinner is the adhesive layer, the higher is the shear strength.

6 76 Surface Treatment 18 OL ) CO BesWil equation: 7.5 ' e ^ ,2 0,3 0,4 Adhesive thickness (mm) 0.5 Figure 5: Effects of the adhesive thickness on the mechanical properties of the adhesive joints 5. Joint's shear tests results Results, reported in table 2, show that the effects of radiation on the mechanical properties of adhesive joints strongly depends on the adherend surface treatment: after a certain dose the joint's shear strength tends to be the same whatever is the utilised surface treatment. Surface treatment Sand Blasting Pickle 1 (see table 1) Dose Shear strength (MPa) 14.5± ± ± ± ±1.2 Table 2: Shear test with gamma irradiation

7 Surface Treatment 77 The joints, after a certain dose which depends on the adherend surface treatment, start to degrade, but the shear strength remains largely sufficient for many applications in high energy physics experiments, even if irradiated up to 4 MGy. 6 Adhesive Tensile Test Results The results, listed in table 3, show that the resin degrade at radiation doses that do not affect joint's shear strength. This can be explained taking into account that a joint can fail both in the adhesive and in the interface. The interface, composed by ionic and physical bonds, is less sensitive to radiation than the adhesive itself. Dose Shear strength (Mpa) 36.5± ± ±2 5 22±4 21.3±2 5 20±4 18.7± Table 3: Dumbbell test with gamma irradiation If the resin has higher shear strength than the interface, of course the joint is failing at the interface. Vice- versa, if the interface is stronger than the polymer, than the joint is failing in the resin, and its strength is the same than the adhesive' s one. For this reason, it is interesting to calculate the adhesive' s shear strength. Shear strength Sgt of a material is related to the tensile strength T# of the same material as demonstrated by the Von Mises Criteria: Adhesives and joints' shear strengths are plotted in figure 6 as functions of radiation dose. As one can see, initially the resin has a higher shear strength than the joints, and the joints' shear strength is not modified by radiation: they are failing at the interface, which is normally resistant to radiation. Vice-versa, the resin is modified by radiation, so that its shear strength decreases with the absorbed dose. When the adhesive 's shear strength becomes smaller than the interface's one, than the joints are also degraded: they are starting to fail in the adhesive.

8 78 Surface Treatment CO CL g) - Joint's shear strength: Pickle 1 Joint's shear strength: Sand Blasting Adhesive's shear strength Radiation Dose (MGy) Figure 6: Adhesive's and joints' shear strengths 7 Conclusions The influence of radiation on the mechanical properties of Araldit AW 106/HV 953 U curing at room temperature epoxy adhesive has been studied. Symmetric single lap shear and dumbbell tests have been utilized. The radiation behavior can be explained taking into account that the adhesive and the interface normally react to radiation in two completely different ways. The interface, dominated by ionic and physical bonds, has a good radiation resistance, but the polymer, composed by covalent bonds, can be strongly modified by radiation. Araldit AW 106/HV 953U demonstrated to be good candidates for different high energy physics applications: shear strength remains largely sufficient for many applications in high energy physics experiments, even if irradiated up to 4 MGy. 8. References F. S. Guarino, C. Hauviller, J. M. Kenny, Proc. Conference, Paris, France, 1998, p. 475 /* Int. SAMPE Europe

9 Surface Treatment 79 3 M. Dole, History ofpolyolefins, R. B. Seymour and T. Cheng, Reidel, Boston, USA, P. Beynel, P. Maier and H. Schonbacher, CERN 82-10, F. S. Guarino, C. Hauviller, J. M. Kenny, in Proc. Int. EUROMECH 358 Conference, Nimes, France, 1997, p R.L. Clough, K.T. Gillen and M. Dole, Radiation Resistance of Polymers and Composites, Irradiation Effects on Polymers\ D.W. Clegg and A.A. Colleyer, 1991, p H.P. Gilfrich and H.Wilski, Radiat Phys Chem, 39 (1992) R. J. Young, P. A. Lovell, Introduction to Polymers, edited by Chapman & Hall, H. Danielsson, CERN/TIS-CFM/IR/90-03, C. Chevalier and M. Tavlet, CERN/TIS-TE/IR/97-03, J. M. Kenny, A. Maffezzoli L. Nicolais, Composite Science and Technology JJ(1990)339