materials

Article of Mo to Glidcop Dispersion Strengthened for Accelerating Structures

Valentina Casalegno 1,*, Sergio Perero 1 , Monica Ferraris 1, Mauro Taborelli 2, Gonzalo Arnau Izquierdo 2, Stefano Sgobba 2 and Milena Salvo 1

1 Applied Science and Technology Department, Institute of Materials Physics and Engineering, Politecnico di Torino, I-10129 Torino, Italy; [email protected] (S.P.); [email protected] (M.F.); [email protected] (M.S.) 2 CERN European Organization of Nuclear Research, CH-1211 Geneva, Switzerland; [email protected] (M.T.); [email protected] (G.A.I.); [email protected] (S.S.) * Correspondence: [email protected]; Fax: +39-011-564-4699

 Received: 18 July 2018; Accepted: 27 August 2018; Published: 7 September 2018 

Abstract: Alumina dispersion-strengthened copper, Glidcop, is used widely in high-heat-load ultra-high-vacuum components for synchrotron light sources (absorbers), accelerator components (beam intercepting devices), and in nuclear power plants. Glidcop has similar thermal and electrical properties to oxygen free electrical (OFE) copper, but has superior mechanical properties, thus making it a feasible structural material; its yield and ultimate tensile strength are equivalent to those of mild-carbon steel. The purpose of this work has been to develop a brazing technique to join Glidcop to Mo, using a commercial Cu-based . The effects of the excessive diffusion of the braze along the grain boundaries on the interfacial chemistry and joint microstructure, as well as on the mechanical performance of the brazed joints, has been investigated. In order to prevent the diffusion of the braze into the Glidcop alloy, a copper barrier layer has been deposited on Glidcop by means of RF-sputtering.

Keywords: brazing; alumina dispersion-strengthened copper; mechanical testing

1. Introduction Glidcop is a family of dispersion strengthened copper alloys, made of a pure copper matrix strengthened by a uniform dispersion of alumina particles. These particles, which are stable at high temperatures and above the melting point of the matrix, have the aim of preventing the softening and recrystallization of the copper when treated at high temperatures [1]. Glidcop exhibits an improved mechanical resistance, especially at high temperatures, with respect to copper, and at the same time it maintains an excellent thermal conductivity. For these reasons, it has been considered, from the very beginning, as a suitable support material for the collimator jaws of modern particle accelerators [2], with the idea that the Glidcop block could be efficiently cooled and would work as a heat sink for the jaw itself. In this sense, it is still the material of choice for the next generation supports of the collimators [3] that have to be installed on the Large Hadron Collider (LHC) during the long shutdown 2. The jaws close to the Glidcop holder are made of a sequence of materials with an increasing atomic number, so that the extremely high energetic proton beam halo is gradually dispersed and finally absorbed. As a consequence, refractory materials, such as , , inermet or similar, are used. In this sense, an effective joint between a highly conducting material and a high Z material could be of interest. The present work focuses on the development of a brazing process for pure Mo and a Glidcop alloy. The authors have already investigated the joining of a CuZr alloy (UNS C15000, 80% cold worked) and Mo in the framework of the Compact Linear Collider (CLIC) project [4]; the accelerating structure is traditionally made of brazed oxygen free electrical (OFE) copper parts,

Materials 2018, 11, 1658; doi:10.3390/ma11091658 www.mdpi.com/journal/materials Materials 2018, 11, 1658 2 of 13 but CuZr and Glidcop could represent improved alternatives for the conducting regions that suffer from to mechanical fatigue. Moreover, the study of the joining of Glidcop can be considered relevant, not only for the particle accelerator components, but also for other application fields, such as resistance welding electrodes, incandescent light bulb leads, hybrid circuit packages and other high temperature applications, such as X-ray tube components or heat exchanger components [5,6]. Several methods have been proposed to join Glidcop to such metals as Cu or stainless steel [7–10]. An assembly of Glidcop cooling pipes, coated with a sacrificial armor in pure W or W alloys has been obtained for the diverter component using TiCuAg alloy as the joining material [6], but to the best of the authors’ knowledge, no joint between Glidcop and Mo has yet been manufactured or characterized. Of all the different joining technologies, the brazing process is known to play the most important role in the joining of Glidcop. Moreover, fusion welding, including electron beam welding, is not suitable for the joining process because the remelting of the copper matrix leads to an agglomeration of the alumina particles and recrystallization of the matrix in the welding area, thus creating a brittle welded zone [11]. The brazing of Glidcop is usually carried out with the brazing filler metals that are commonly used to join plain copper (i.e., - and -based braze alloys). The brazing of copper generally leads to a grain coarsening, due to the grain growth that occurs during the brazing process. Glidcop does not suffer from this problem, because of its fine structure and reduced recrystallization at the brazing temperatures. The main problem related to the brazing of Glidcop is the excessive diffusion of the silver-based filler along the grain boundaries of Glidcop. The of Glidcop with copper or prior to brazing is used as a common method to prevent and solve this issue [10]. Copper plating is usually carried out in a copper cyanide solution; the cyanide-copper bath has the aim of facilitating the quality of the braze joints by preventing diffusion of the braze alloy into the Glidcop base material. On the other hand, the Cu-plating process introduces additional steps in the manufacturing process, since additional control stages are needed to ensure blister-free plating. Nickel plating is obtained through more complicated processes, which are based on a Watts bath or electroplating. The purpose of this work has been to develop a brazing technique to join Glidcop to Mo using a commercial silver-free Cu-based alloy. In order to prevent the diffusion of the braze into the Glidcop, a copper barrier layer has been deposited on the Glidcop by means of RF-sputtering, an environmentally friendly coating technique, which does not require the use of any hazardous chemicals.

2. Materials and Methods Glidcop is a made up of oxygen free copper and aluminum oxide; it consists of more than 99% copper. The Glidcop Grade used in this work is Glidcop AL-25, in which the aluminum oxide content is 0.5 wt.%; the commercial name is UNS-C15725, and the Al2O3 submicroscopic dispersed particle size ranges from 3 to 12 nm; it was supplied by Höganäs AB (Höganäs, Sweden), while the Mo (99.97% purity) was supplied by Plansee (Gemeinde Reutte, Austria). The joining process between Glidcop and Mo was performed using a silver-free commercial metal braze (Gemco), which does not contain elements such as Ti, Si, etc. that could lead to the formation of brittle intermetallics with Cu. The braze was supplied by Wesgo Metals, and it is composed of 87.75 wt.% Cu, 12 wt.% Ge and 0.25 wt.% Ni; it has a liquidus temperature at 975 ◦C and a solidus temperature at 880 ◦C. The braze foil thickness is about 60 µm. The wettability of the brazing alloy on the Glidcop alloy was measured by means of a hot stage microscope (model AII, Leitz GmbH, Wetzlar, Germany), up to 1000 ◦C under flowing Ar, at a heating rate of 20 ◦C·min−1, equipped with a Leica DBP camera (Ernst Leitz GMBH, Wetzlar, Germany). The adherends were cut using a diamond-grinding disc, and this was followed by grinding with 2500 grit SiC paper. The final polishing was performed using a 3 µm diamond paste. The samples were then cleaned in an ultrasonic bath with acetone for 5 min to remove surface impurities. A Cu coating was deposited on the Glidcop plates for a certain set of samples, by means of Radio Frequency Materials 2018, 11, 1658 3 of 13 magnetron sputtering, before the brazing process. A copper target (99.99% purity), supplied by Franco Corradi (Rho (MI), Italy), was used. The sputtering deposition parameters were varied in order to optimize the homogeneity of the coating. A pure Ar (99.996% purity) atmosphere was used to avoid oxidization of the sputtered layer during deposition. The cathode-substrate distance was maintained fixed at 14 cm. The pre-deposition pressure reached 7.0 × 10−5 Pa, while the Ar pressure was 55 × 10−1 Pa during deposition. A power of 250 W was applied in RF (radio frequency), to a 6 inch diameter target. This value was selected, after some preliminary depositions, in order to balance the deposition rate, which needed to be as high as possible, and the thin film morphology. Low power can lead to voids and defects in the layer, while high power can create other unwanted structures, such as columns and recrystallization. The film thickness was controlled while varying the deposition time. Different time periods (from 20 min to 7 h) were considered to obtain Cu coatings of various thickness (from 1 µm to 18 µm). The thickness measurements were performed by means of a surface profiler (Tenkor P11, KLA-Tenkor, Milpitas, CA, USA). The homogeneity area of the deposition surface was around 100 mm2. The produced joints were sandwich-like Mo/Gemco/Glidcop. The metallic parts that had to be joined and the brazing alloy foil were sectioned into 5 mm × 10 mm pieces, to obtain a joining surface of 50 mm2. One foil or three foils of the Gemco brazing alloy were used. The brazing was performed in a horizontal tube furnace (BICASA, Camera SUPERTHAL, Bernareggio (MB), Italy) under flowing Ar; the brazing temperature was chosen slightly above the filler metal liquidus temperature (980 ◦C), and the heating rate adopted to reach the brazing temperature was 1000 ◦C·h−1. The dwelling time was varied from 1 min to 30 min; the specimens were then allowed to cool to room temperature in the furnace (cooling rate of 5 ◦C min−1). The influence of a tungsten weight (100 g, corresponding to 0.02 MPa nominal pressure) on the top of the Mo/Glidcop sandwich structure was also investigated. This applied load is about ten times the specimens’ own mass. The microstructure of the joined samples was investigated by means of optical and electron microscopy (SEM, Philips 525 M, FEI_thermo Fisher Scientific, Hillsboro, OR, USA, accelerating voltage = 15–30 kV) coupled with energy dispersive spectroscopy for the compositional analysis (EDS SW9100 EDAX and Oxford Isis 300, EDAX-AMETEK Materials Analysis Division, Meerbusch, Germany). The apparent shear strength of the joints was measured with a single-lap test under compression, adapted from ASTM D905-08 standard [12], specimen size: 5 × 10 × 3 mm, seen in Figure1B, and according the ASTM B898-11 standard [13] (Glidcop size of 25 × 15 × 10 mm and Mo size of 4 × 15 × 4 mm), seen in Figure1A, at room temperature (universal testing machine SINTEC D/10, MTS Systems, Eden Prairie, Minnesota, USA). The shear strength was determined as the ratio of the load measured at the fracture and the joined area. The compressive force was applied at a speed of 0.5 mm·s−1. The fracture surfaces were examined to determine the fracture propagation. Rockwell B hardness measurements were performed on the Glidcop surface, before and after the brazing process, using an Officine Galileo hardness tester (Durometro Galileo A200, Rockwell, Officine Galileo, Capalle (FI), Italy). Materials 2018, 11, 1658 4 of 13 Materials 2018, 11, x FOR PEER REVIEW 4 of 13 Materials 2018, 11, x FOR PEER REVIEW 4 of 13

Figure 1．Configuration for measuring the apparent shear strength of Mo/Glidcop joined samples: Figure 1．Configuration for measuring the apparent shear strength of Mo/Glidcop joined samples: (AFigure) ASTM 1. B898-11Configuration standard for (Glidcop measuring size the of apparent25 × 15 × shear10 mm strength and Mo of size Mo/Glidcop 4 × 15 × 4 joinedmm) and samples: (B) (A) ASTM B898-11 standard (Glidcop size of 25 ×× 15 ×× 10 mm and Mo size 4 × ×15 × 4× mm) and (B) single-lap(A) ASTM test, B898-11 adapted standard from ASTM (Glidcop D905- size08 of(adherends 25 15 size:10 5 mm × 10 and × 3 Momm). size 4 15 4 mm) and (single-lapB) single-lap test, test, adapted adapted from from ASTM ASTM D905- D905-0808 (adherends (adherends size: size: 5 5 ×× 1010 ×× 3 mm).3 mm). 3. Results and Discussion 3. Results and Discussion The wettability of the brazing alloy was only measured on the Glidcop substrate, in order to The wettability of the brazing alloy was only measured on the GlidcopGlidcop substrate, in order to demonstrate the compatibility and spreading of the braze on Glidcop; the braze wettability on Mo demonstrate thethe compatibilitycompatibility and and spreading spreading of of the the braze braze on on Glidcop; Glidcop; the the braze braze wettability wettability on Mo on hadMo had already been presented and discussed elsewhere [4]; moreover, the wetting of the Cu-Mo system alreadyhad already been been presented presented and discussedand discussed elsewhere elsewhere [4]; moreover,[4]; moreover, the wettingthe wetting of the of Cu-Mothe Cu-Mo system system is a is a well-known and broadly studied topic [14]. The wettability test showed that the molten alloy well-knownis a well-known and broadlyand broadly studied studied topic topic [14]. The[14]. wettabilityThe wettability test showed test showed that the that molten the molten alloy startsalloy starts spreading at◦ 920 °C and at◦ 980 °C the contact angle between the molten Gemco and Glidcop spreadingstarts spreading at 920 atC 920 and °C at 980and Cat the980 contact °C the anglecontact between angle thebetween molten the Gemco molten and Gemco Glidcop and reaches Glidcop its reaches its lowest value, as seen in Figure 2. An equilibrium configuration is reached almost lowestreaches value, its lowest as seen value, in Figure as 2seen. An equilibriumin Figure 2. configuration An equilibrium is reached configuration almost immediately, is reached thatalmost is, immediately, that is, after◦ one min at 980 °C; after 15 min of contact, no variation in the spreading afterimmediately, one min atthat 980 is, C;after after one 15 min min at of 980 contact, °C; after no variation 15 min of in contact, the spreading no variation behavior in wasthe spreading observed. behavior was observed. behavior was observed.

FigureFigure 2. 2. Hot stagestage microscope microscope images images of aof Gemco a Gemco braze foilbraze on Glidcopfoil on substrate;Glidcop substrate; the best spreadability the best Figure 2. Hot stage◦ microscope images of a Gemco ◦braze foil−1 on Glidcop substrate; the best spreadabilitywas reached was at 980 reachedC (under at 980 flowing °C (under Ar, heatingflowing rateAr, 20heatingC·min rate 20). °C·min−1). spreadability was reached at 980 °C (under flowing Ar, heating rate 20 °C·min−1).

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As far as thethe brazingbrazing process isis concerned,concerned, aa preliminarypreliminary studystudy was carried out to optimizeoptimize the thicknessthickness of thethe jointjoint andand itsits microstructure.microstructure. The joint gap was controlled by the thicknessthickness of thethe brazing fillerfiller foil; a reduction inin the joint thickness can be helpful to obtain a joint with good electrical ◦ propertiesproperties [[4].4]. The brazing treatmenttreatment waswas performedperformed atat 980980 °C,C, that is, at a temperature slightly above thethe liquidus of the fillerfiller alloy, inin orderorder toto reachreach thethe bestbest spreadabilityspreadability andand wettabilitywettability ofof thethe brazebraze and toto avoidavoid thethe recoveryrecovery andand thethe softeningsoftening ofof thethe coppercopper alloyalloy [[8,9].8,9]. Figure3 3 shows shows a a SEM SEM cross-section cross-section of of an an Mo/Gemco/Glidcop Mo/Gemco/Glidcop jointjoint obtainedobtained usingusing oneone brazebraze foilfoil ◦ (at(at 980980 °CC for 5 min, under flowingflowing ArAr andand applyingapplying 0.020.02 MPa).MPa).

Figure 3.3. SEM micrographmicrograph ofof thethe cross-sectioncross-sectionof ofa a Mo/Gemco/Glidcop Mo/Gemco/Glidcop joint manufactured by using one Gemco braze foilfoil atat 980980 ◦°C,C, 5 min, underunder flowingflowing Ar and applyingapplying 0.020.02 MPa.MPa.

The thickness of the braze metal, after the thermalthermal treatment in the samplessamples joined with one brazing alloy foil, waswas aboutabout 4040 µμm.m. Because Because of the high wettability of the brazing alloy on the substrates, the braze can flowflow out of the joint gap, thus justifying the reduction in braze thickness. Several voids in the fillerfiller metal layer were visiblevisible andand the interface waswas notnot continuous.continuous. On the other hand, there is littlelittle evidenceevidence ofof significantsignificant defects,defects, suchsuch asas micro-voids,micro-voids, in thethe jointsjoints manufacturedmanufactured using three Gemco braze foils, shown in Figure4 4,, duedue toto brazebraze solidificationsolidification shrinkage,shrinkage, andand bothboth MoMo and Glidcop exhibit a good metallurgical continuity in the interfacial region. Moreover, a comparison between twotwo Mo/Glidcop Mo/Glidcop joints joints obtained obtained with with three three Gemco Gemco foils foils at 980 at◦ C,980 5 min,°C, 5 but min, with but and with without and awithout superimposed a superimposed load (100 load g, corresponding (100 g, corresponding to 0.02 MPa to 0.02 nominal MPa pressure), nominal pressure), was carried was out. carried When out. the pressureWhen the was pressure not applied was not to applied the assembly, to the aassembly, higher number a higher of number voids developed, of voids developed, due to the due shrinkage to the ofshrinkage the liquid of brazethe liquid (pictures braze not (pictures reported not here); reported then, he there); introduction then, the introduction of an external of an load external during load the brazingduring the process brazing was process able to reducewas able these to reduce voids. Fromthese avoids. qualitative From a observation qualitative ofobservation the cross-section of the ofcross-section the samples, of the the surface samples, corresponding the surface to corresponding voids is reduced to byvoids 80%. is Further reduced investigations by 80%. Further were addressedinvestigations to optimize were addressed the dwelling to time; optimize all the the different dwelling experimental time; all parameters the different are summarized experimental in Tableparameters1. are summarized in Table 1.

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Figure 4.4. SEMSEM micrographmicrograph ofof thethe cross-sectioncross-sectionof of a a Mo/Gemco/Glidcop Mo/Gemco/Glidcop joint manufactured by using threethree GemcoGemco brazebraze foilsfoils atat 980980 ◦°C,C, 5 min, underunder flowingflowing ArAr andand applyingapplying 0.020.02 MPa.MPa.

◦ TableTable 1.1.Brazing Brazing parameters parameters for for several several sets sets of of samples; sample joinings; joining temperature temperature fixed fixed at 980 at 980C, heating°C, heating rate ◦ 1000rate 1000C/h, °C/h, Ar atmosphere Ar atmosphere (100 g superimposed(100 g superimpos weighted correspondsweight corresponds to 0.02 MPa to nominal0.02 MPa pressure). nominal

pressure). Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 7 Sample 8 Number of Brazing Foils 3 3 3 1 1 3 3 3 Set up Superimposed WeightSample (100 g) yesSample yesSample noSample yesSample no Sample yes Sample yes Sample yes Process parameters Dwelling Time (min)1 12 53 54 55 56 10 7 15 308

Number of 3 3 3 1 1 3 3 3 In order toBrazing study Foils the distribution of the elements across the joint region, EDS analyses were Set up performed on theSuperimposed Mo/Gemco/Glidcop samples brazed using three braze foils (best morphological yes yes no yes no yes yes yes result, accordingWeight to the (100 microstructure g) shown in Figure4) at 980 ◦C for 1, 5, 10, 15, 30 min and applying 0.02Process MPa (picturesDwelling not Time reported here). Germanium diffused extensively from the braze to the Glidcop 1 5 5 5 5 10 15 30 substrateparameters in all the(min) samples. The Ni content could not be determined because its concentration was too low (0.25 wt.% of Ni in the brazing alloy). FurtherIn order investigations,to study the distribution by means ofof SEM,the elements allowed across an excessive the joint diffusion region, EDS of the analyses braze towere be detectedperformed along on the grainMo/Gemco/Glidcop boundaries of the samples Glidcop braz alloy,ed asusing it is possiblethree braze to observe foils (best in the morphological micrographs reportedresult, according in Figure 5to for the samples microstructure brazed at shown 980 ◦C in for Fi bothgure 54) and at 30980 min °C usingfor 1, three 5, 10, Gemco 15, 30 alloy min foilsand andapplying applying 0.02 0.02MPa MPa. (pictures not reported here). Germanium diffused extensively from the braze to the Glidcop substrate in all the samples. The Ni content could not be determined because its concentration was too low (0.25 wt.% of Ni in the brazing alloy). Further investigations, by means of SEM, allowed an excessive diffusion of the braze to be detected along the grain boundaries of the Glidcop alloy, as it is possible to observe in the micrographs reported in Figure 5 for samples brazed at 980 °C for both 5 and 30 min using three Gemco alloy foils and applying 0.02 MPa.

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FigureFigure 5. 5. SEMSEM micrographs micrographs of of the the cross-section cross-section of of a Glidcop/Mo interface interface obtained obtained by by means means of of brazingbrazing (three (three Gemco Gemco braze braze foils) foils) at at 980 980 °C◦C for for 5 5 min min ( (AA)) and and for for 30 30 min min ( (BB)) under under flowing flowing Ar Ar and and applyingapplying 0.02 0.02 MPa. MPa.

ThisThis result result is isconsistent consistent with with the common the common diffusion diffusion theory theoryand literature and literature references. references. Several studiesSeveral [15] studies have [reported15] have that, reported during that, the during brazing the of Glidcop brazing with of Glidcop gold or with silver gold alloys or (mainly silver alloys Au- Cu(mainly or Ag-Cu Au-Cu based or Ag-Cu alloys), based there alloys), is a tendency there is aof tendency the gold ofor the silv golder to or diffuse silver tointo diffuse the Glidcop, into the dependingGlidcop, depending on the duration on the of duration the brazing of thecycle. brazing To the cycle. best of To the the authors’ best of knowledge, the authors’ no knowledge, studies on Geno and studies Ni diffusion on Ge and in Ni DS-copper diffusion alloys in DS-copper are available alloys in are literature. available in literature. TheThe brazed brazed area area can can be be divided divided into into the the braze braze me metaltal layer layer and and the the diffusion diffusion layer, layer, in in which which the the liquidliquid metal metal migrates migrates along along the the Glidcop Glidcop grain grain boundaries, boundaries, as as indicated indicated in in Figure Figure 55.. In thethe BSE-SEMBSE-SEM imagesimages shown shown in in Figure Figure 55,, thethe brazebraze waswas foundfound toto migratemigrate alongalong thethe GlidcopGlidcop graingrain boundariesboundaries forfor severalseveral hundreds hundreds of of microns, microns, thus thus determining determining the the formation formation of of a a significant significant porosity porosity in in the the Glidcop Glidcop alloy;alloy; there there is is evidence evidence that that the the number number of of voids voids at at the the grain grain boundaries boundaries increases increases slightly slightly and and the the voidsvoids grow inin size size (qualitative (qualitative observation, observation, void void ratio/size ratio/size not investigated)not investigated) on the on basis the ofbasis the brazingof the brazingprocess dwellingprocess dwelling time (the time effect (the of theeffect temperature of the temperature has not been has studied not been here). studied This phenomenon here). This phenomenoncan be explained can be by explained considering by theconsidering coalescence the ofco thealescence pre-existing of the voidspre-existing at the grainvoids boundaryat the grain or boundaryby the depletion or by the of depletion elements (i.e.,of elements Ge) from (i.e., the Ge brazing) from alloythe brazing that penetrates alloy thatthe penetrates grain boundaries. the grain boundaries.According toAccording the Ge-Cu to phasethe Ge-Cu diagram phase [16 diagram], the Gemco [16], alloythe Gemco is at a alloy completely is at a completely liquid state liquid at the statebrazing at the temperature, brazing temperature, and this allows and this the allows formation the formation of a thick of diffusion a thick layer.diffusion Only layer. the Only diffusion the diffusionlayer can layer be detected can be fromdetected the morphologicalfrom the morphologi analysis,cal and analysis, it clearly and exceeds it clearly the exceeds expected the filler expected metal fillerlayer metal thickness layer (about thickness 180 µ m).(about According 180 µm). to FigureAccording5, the diffusionto Figure distance 5, the indiffusion Glidcop/Mo distance joints in Glidcop/Moobtained using joints three obtained Gemco using braze three foils atGemco 980 ◦C braze for 5 foils min andat 980 for °C 30 for min 5 min is about and 250for µ30m min and is 350 aboutµm, 250respectively. µm and 350 As µm, in therespectively. case of silver-based As in the case brazes, of silver-based the rate of brazes, diffusion the rate of Gemco of diffusion along of the Gemco grain alongboundaries the grain of Glidcop boundaries is higher of Glidcop than its rateis higher of diffusion than its through rate of the diffusion grains; asthrough a consequence, the grains; there as isa consequence,a preferential there diffusion is a preferential or migration diffusion of Gemco or along migration the grain of Gemco boundaries along and the agrain successive boundaries formation and aof successive voids at the formation grain boundaries. of voids at Conversely, the grain boundari the diffusiones. Conversely, rate of the the elements diffusion contained rate of inthe the elements brazing containedalloys along in the brazing grain boundaries alloys along is significantthe grain boundaries in Glidcop, is and significant this allows in Glidcop, a rapid migrationand this allows of the aelements rapid migration inside the of materialsthe elements and inside a depletion the ma ofterials the joining and a area.depletion of the joining area. ItIt has has been been reported reported that that fine fine grained grained Glidcop, Glidcop, which which have have se severalveral grain grain boundaries, boundaries, permits permits a a rapidrapid diffusion diffusion of of the the constituents constituents of of the the braze braze allo alloy,y, particularly particularly Ag, Ag, along along the the grain grain boundaries boundaries [7]. [7]. Moreover,Moreover, this this excessive excessive diffusion diffusion of of elements elements al alongong the the grain grain boundaries boundaries of of Glidcop Glidcop causes causes the the formationformation of of small small voids voids near near the the braze braze area. area. Therefore, Therefore, in in order order to to limit limit the the diffusion diffusion phenomena phenomena of of GeGe and and Cu, Cu, the the dwelling dwelling time time was was reduced reduced to to 1 1 min; min; as as a a consequence, consequence, the the joining joining process process was was carried carried outout at at 980 980 °C,◦C, for for only only 1 1 min, using three braze layers and applying 0.02 MPa. The The morphological analysisanalysis (not reported here)here) ofof the the cross cross section section of of the the Mo/Glidcop Mo/Glidcop joints joints manufactured manufactured according according to theto the aforementioned parameters does not differ significantly from that observed for the 5 min dwelling time for the brazing process, as seen in Figure 4. Consequently, to overcome the braze

Materials 2018, 11, 1658 8 of 13 Materials 2018, 11, x FOR PEER REVIEW 8 of 13 aforementioneddiffusion along the parameters Glidcop doesgrain not bo differundaries, significantly the Glidcop from surface that observed was coated for the with 5 min pure dwelling Cu prior time to forthe thebrazing brazing process. process, as seen in Figure4. Consequently, to overcome the braze diffusion along the GlidcopCopper grain and boundaries, nickel coatings the Glidcop are well surface known was barrier coated layers with to pure the Cu diffusion prior to of the metallic brazing elements process. into Glidcop;Copperand the nickelminimum coatings required are wellcoating known thickn barrieress for layers a brazing to the treatment diffusion is of usually metallic a function elements intoof the Glidcop; brazing the temperature minimum and required the brazing coating time. thickness for a brazing treatment is usually a function of the brazingFurthermore, temperature the adherence and the brazingof the coating time. to the substrate plays an important role. A Watts bath or nickelFurthermore, sulphamate the bath adherence are genera of thelly coatingused for to the the nickel substrate plating, plays and an the important grain size role. and A orientation Watts bath orof the nickel plated sulphamate layers can bath influence are generally the barrier used function for thenickel of the plating,plating. andA cyanide the grain bath size or an and acid orientation copper ofsulfate the plated bath are layers currently can influence used for the copper barrier plating function [10,17 of]. the plating. A cyanide bath or an acid copper sulfateInstead bath areof these currently traditional used for techniques, copper plating we performed [10,17]. the Cu plating of Glidcop by means of RF- sputtering.Instead The of theseprocess traditional has a fast techniques, deposition we rate performed, and the thethickness Cu plating of the of coating Glidcop can by easily means be of RF-sputtering.controlled and tailored The process on the has basis a fast of depositionthe brazing rate,parameters. and the thickness of the coating can easily be controlledThis coating and tailored process on is the a complete basis of thedry brazingand environm parameters.ental friendly technique; it is a good process to obtainThis highly coating reproducible process is a completesurface quality, dry and without environmental employing friendly environmentally technique; itharmful is a good and process toxic tosubstances, obtain highly thus reproduciblemeeting the surfaceRegistration, quality, Evalua withouttion, employing Authorisation environmentally and Restriction harmful of Chemicals and toxic substances,(EU REACh) thus regulations meeting [18]. the Registration, Preliminary Evaluation,experimentsAuthorisation were carried out and with Restriction a 1 µm of Cu Chemicals coating (EUsputtered REACh) onto regulations Glidcop; the [18 metallographic]. Preliminary examination experiments of were the carriedjoined Glidcop/Gemco/Mo out with a 1 µm Cu interface coating sputtereddid not show onto any Glidcop; decohesion the metallographic at the Cu sputtered/ examinationGlidcop of interface, the joined and Glidcop/Gemco/Mo defect-free Gemco/substrate interface didinterfaces not show were any observed, decohesion as seen at the in Cu Figure sputtered/Glidcop 6. On the other interface, hand, the and specimen defect-free still Gemco/substrateexhibited strong interfacesintergranular were penetration observed, of as the seen braze in Figure along6 the. On Glidcop the other grain hand, boundaries the specimen (about still 200 exhibited µm through strong the intergranularthickness). An penetration EDS analysis of the at brazethe grain along boundaries the Glidcop in grain Glidcop, boundaries at about (about 150 200 µmµ mfrom through the theGemco/Glidcop thickness). Aninterface, EDS analysisseen in Figure at the grain6B, showed boundaries the presence in Glidcop, of Ge-enriched at about 150 grainµm boundary from the Gemco/Glidcopregions in the Glidcop. interface, As a seen conclusion, in Figure the6B, 1 showedµm Cu sputtered the presence coating of Ge-enriched was considered grain as boundarynot being regionseffective in against the Glidcop. the diffusion As a conclusion, of Gemco in the Glidcop. 1 µm Cu sputtered coating was considered as not being effective against the diffusion of Gemco in Glidcop.

Figure 6. ScanningScanning electron electron micrographs of ( A)) the the cross-section cross-section of of 1-µm 1-µm thick thick Cu- Cu- plated plated Glidcop/Mo Glidcop/Mo joint;joint; (B) magnificationmagnification of a grain boundary boundary in in the the Glidcop Glidcop and and relative relative EDS EDS analysis analysis (inset) (inset) at at about about 150150 micronsmicrons fromfrom Glidcop/GemcoGlidcop/Gemco interface. interface.

To corroborate this statement, the distribution of the considered elements, obtained from an EDS analysis across the joint region, is shown in Figure7. The diagram is scaled with respect to the distribution of the elements at the Glidcop/Mo interface. The main braze constituents diffused significantly in the Glidcop up to 600 µm from the interface. On the other hand, Ge-Mo intermetallic phases were able to form at the interface between the Gemco and Mo, according to the phase diagram. Moreover, the EDS analysis detected Ge (up to 4%).

Materials 2018, 11, x FOR PEER REVIEW 9 of 13

To corroborate this statement, the distribution of the considered elements, obtained from an EDS analysis across the joint region, is shown in Figure 7. The diagram is scaled with respect to the distribution of the elements at the Glidcop/Mo interface. The main braze constituents diffused significantly in the Glidcop up to 600 µm from the interface. On the other hand, Ge-Mo intermetallic Materials 2018phases, 11 ,were 1658 able to form at the interface between the Gemco and Mo, according to the phase diagram. 9 of 13 Moreover, the EDS analysis detected Ge (up to 4%).

Figure 7. µ FigureElement 7. Element distribution distribution (Ge, Cu(Ge, and Cu and Mo) Mo) across across the the 1- 1-µmm thick thick Cu Cu plated plated Glidcop/braze/Mo Glidcop/braze/Mo joint. joint. The same analysis was carried out on the joints obtained with a thicker (18 µm) Cu sputtered layer on Glidcop;The this same value analysis was chosenwas carried according out on the to joints the values obtained reported with a thicker in literature (18 µm) forCu electrochemicalsputtered platinglayer (minimum on Glidcop; 15–20 this micron). value was Figure chosen8 shows according the to cross-section the values reported of a 18- inµ mliterature thick Cufor plated Glidcop/Gemco/Moelectrochemical plating joint; (minimum no voids 15–20 are detectablemicron). Figure at the8 shows braze/adherend the cross-section interface of a 18-µm or thick along the Cu plated Glidcop/Gemco/Mo joint; no voids are detectable at the braze/adherend interface or along Glidcop grain boundaries, thus demonstrating that the thicker copper coating successfully limited the the Glidcop grain boundaries, thus demonstrating that the thicker copper coating successfully limited diffusionthe of diffusion elements of fromelements the brazefrom the along braze the along grain the boundaries grain boundaries of Glidcop of Glidcop and allowedand allowed the the formation of a defect-freeformation brazed of a defect-free joint. The brazed EDS joint. element The EDS maps elem reportedent maps inreported Figure in8 B–DFigure demonstrate 8B–D demonstrate that the Ge was confinedthat the to Ge the was braze confined alloy to the (about braze 180 alloyµ m(about thick) 180 and µm thick) did not and diffuse did not intodiffuse the into Glidcop the Glidcop alloy. Materialsalloy. 2018, 11, x FOR PEER REVIEW 10 of 13

A B

C D

Figure 8.Figure(A)SEM 8. (A)SEM cross-section cross-section of anof an 18- 18-µmµm thick thick CuCu plated Glidcop/Mo Glidcop/Mo joint; joint; (B) Cu (B EDS) Cu mapping; EDS mapping; (C) Ge EDS mapping; (D) Mo EDS mapping. (C) Ge EDS mapping; (D) Mo EDS mapping. In short, the excessive reduction of the filler in the Glidcop, and the consequent formation of detrimental voids in the alloy, can be avoided by depositing a Cu diffusion barrier coating by means of RF-sputtering. Moreover, unlike the commonly adopted electroplating process, this is a simple process that does not require chemicals or lead to blister formation at the plating/substrate interface. In conclusion, the optimal brazed specimens were manufactured at 980 °C for 1 min, under flowing Ar by using three Gemco foils as brazing material, after having Cu-plated the Glidcop surface by an 18 µm Cu layer by RF-sputtering. As a consequence, the optimized joints are four components sandwiched, made of Glidcop, thin Cu layer, braze foils, and Mo block.

Mechanical Tests Some joints, manufactured using the optimized process were mechanically tested in a single-lap test to determine the apparent shear strength. The fracture did not occur in the joined area, but all the tested samples detached from the fixture, seen Figure 1B; the glue failed for stresses of up to 42 MPa, and no fracturing occurred in the joined samples. Unlike what was observed where a CuZr alloy was joined to Mo using the same braze [4], no plastic deformation occurred in Glidcop, but, since no failure of the joint was observed, this test configuration could not be used to test the apparent shear strength of these joints. As a consequence, other brazed specimens were manufactured, according to the ASTM B898-11 standard, seen in Figure 1A, and an average value of 68.5 ± 18.0 MPa was measured. The fracture surface analysis shows that cracks occurred preferentially in the braze and/or at the braze/Glidcop interface, but not at the braze/Mo interface. Figure 9 shows the typical fracture surfaces for a Mo/Glidcop joined sample: the presence of the brazing alloy is evident on both sides.

Materials 2018, 11, 1658 10 of 13

In short, the excessive reduction of the filler in the Glidcop, and the consequent formation of detrimental voids in the alloy, can be avoided by depositing a Cu diffusion barrier coating by means of RF-sputtering. Moreover, unlike the commonly adopted electroplating process, this is a simple process that does not require chemicals or lead to blister formation at the plating/substrate interface. In conclusion, the optimal brazed specimens were manufactured at 980 ◦C for 1 min, under flowing Ar by using three Gemco foils as brazing material, after having Cu-plated the Glidcop surface by an 18 µm Cu layer by RF-sputtering. As a consequence, the optimized joints are four components sandwiched, made of Glidcop, thin Cu layer, braze foils, and Mo block.

Mechanical Tests Some joints, manufactured using the optimized process were mechanically tested in a single-lap test to determine the apparent shear strength. The fracture did not occur in the joined area, but all the tested samples detached from the fixture, seen Figure1B; the glue failed for stresses of up to 42 MPa, and no fracturing occurred in the joined samples. Unlike what was observed where a CuZr alloy was joined to Mo using the same braze [4], no plastic deformation occurred in Glidcop, but, since no failure of the joint was observed, this test configuration could not be used to test the apparent shear strength of these joints. As a consequence, other brazed specimens were manufactured, according to the ASTM B898-11 standard, seen in Figure1A, and an average value of 68.5 ± 18.0 MPa was measured. The fracture surface analysis shows that cracks occurred preferentially in the braze and/or at the braze/Glidcop interface, but not at the braze/Mo interface. Figure9 shows the typical fracture surfaces forMaterials a Mo/Glidcop 2018, 11, x FOR joined PEER sample: REVIEW the presence of the brazing alloy is evident on both sides. 11 of 13

FigureFigure 9. 9. FractureFracture surface of of Mo/Glidcop Mo/Glidcop brazed brazed samples samples after after apparent apparent shear shear test according test according to ASTM to ASTMB898-11. B898-11.

TheThe fracture fracture mechanism mechanism involves involves coalescence coalescence of of the the microvoids, microvoids, seen seen in in Figure Figure 10 10,, as as in in a a typical typical ductileductile failure. failure. Furthermore, Furthermore, the the EDS EDS analysis analysis performed performed on on all all the the areas areas marked marked in in Figure Figure 10 10 only only revealedrevealed the the presence presence of of Cu Cu and and Ge Ge contained contained in in the the brazing brazing alloy alloy on on both both of of the the fracture fracture surfaces; surfaces; thethe relative relative amount amount of of these these elements elements points points out out that that the the failure failure occurred occurred mainly mainly within within the the braze, braze, thusthus indicating indicating a a lower lower joint joint strength strength between between the the braze braze and and the the Glidcop Glidcop than than the the shear shear strength strength of of thethe dispersion-strengthened dispersion-strengthened alloy alloy itself. itself. The The cohesive cohesive behavior behavior of of the the joined joined samples samples after after testing testing demonstratesdemonstrates the the good good adhesion adhesion at at the the Mo/braze Mo/braze and and Glidcop/braze Glidcop/braze interface.interface.

A B C

D E F

Figure 10. SEM and EDS analysis on Glidcop and Mo fracture surface apparent shear test according to ASTM B898-11; on both surfaces only Cu and Ge have been detected, thus indicating that cracks propagate within the braze and in some areas close to or in Glidcop; the EDS spectrum refers to measurements done in all the orange areas. (A) fracture surface of Glidcop side; (B) fracture surface of Mo side; (C) EDS analysis on orange areas of fracture surfaces shown in (A,B); (D) magnification of fracture surface area on Glidcop side; (E) magnification of fracture surface area on Mo side; (F) higher magnification of area shown in (E).

Materials 2018, 11, x FOR PEER REVIEW 11 of 13

Figure 9. Fracture surface of Mo/Glidcop brazed samples after apparent shear test according to ASTM B898-11.

The fracture mechanism involves coalescence of the microvoids, seen in Figure 10, as in a typical ductile failure. Furthermore, the EDS analysis performed on all the areas marked in Figure 10 only revealed the presence of Cu and Ge contained in the brazing alloy on both of the fracture surfaces; the relative amount of these elements points out that the failure occurred mainly within the braze, thus indicating a lower joint strength between the braze and the Glidcop than the shear strength of the dispersion-strengthened alloy itself. The cohesive behavior of the joined samples after testing Materialsdemonstrates2018, 11, the 1658 good adhesion at the Mo/braze and Glidcop/braze interface. 11 of 13

A B C

D E F

FigureFigure 10.10. SEMSEM andand EDSEDS analysis on Glid Glidcopcop and and Mo Mo fracture fracture surface surface a apparentpparent shear shear test test according according toto ASTMASTM B898-11;B898-11; onon bothboth surfaces only Cu Cu and and Ge Ge have have been been detected, detected, thus thus indicating indicating that that cracks cracks propagatepropagate withinwithin the braze and in some areas close close to to or or in in Glidcop; Glidcop; the the EDS EDS spectrum spectrum refers refers to to measurementsmeasurements donedone inin allall thethe orangeorange areas.areas. (A(A)) fracture fracture surface surface of of Glidcop Glidcop side; side; ( B(B)) fracture fracture surface surface of Moof Mo side; side; (C )( EDSC) EDS analysis analysis on on orange orange areas areas of of fracture fracture surfaces surfaces shown shown in in (A (A,B,);B); (D (D) magnification) magnification of fractureof fracture surface surface area area on Glidcopon Glidcop side; side; (E) magnification(E) magnification of fracture of fracture surface surface area onarea Mo on side; Mo (side;F) higher (F) magnificationhigher magnification of area shownof area inshown (E). in (E).

Hardness tests were carried out in order to investigate the influence of the brazing process on the Glidcop properties; the hardness values of the Glidcop, before and after the brazing process, were 70 ± 2 Hardness Rockwell B (HRB) and 74 ± 1 HRB, respectively. These data show that the brazing process does not result in a significant change in the mechanical properties of the Glidcop, relative to its hardness.

4. Conclusions The joining of Glidcop AL-25 and Mo has been investigated in detail. A brazing process that involved a commercial Cu-based alloy (Ag free) was studied; it led to microstructurally continuous joints with good shear strength. In light of the results of this investigation, the joining parameters were then optimized and Cu plating, by means of sputtering, was used to prevent diffusion of the brazing elements across the joint interface. Unlike the commonly adopted electroplating process, this is a simple process that does not require chemicals or lead to blister formation at the plating/substrate interface. The optimization of the joining process was based on the number of braze foils and on the study of diffusion of the brazing elements in Glidcop. The sputtering of a copper diffusion barrier on Glidcop, to prevent diffusion of the braze, was effective in reducing but not in completely eliminating the formation of voids and cracks, as a result of the preferential migration of braze along the Glidcop grain boundaries. The optimal brazed specimens were obtained at 980 ◦C, 1 min, under flowing Ar, using 3 Gemco foils as brazing material, after having Cu-plated the Glidcop surface by a 18 µm Cu layer by RF-sputtering. The apparent shear strength (adapted from ASTM B898-11) on the optimized joined samples was about 68 MPa. Materials 2018, 11, 1658 12 of 13

Author Contributions: V.C. performed experimental analysis and interpretation of data and contributed in writing original draft; S.P. contributed in experimental activity; M.F. contributed in supervision and validation of the activity; M.T. contributed in revision of the draft and analysis of data; G.A.I. and S.S. contributed to the conception of the work; M.S. contributed equally as first author. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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